The name sounds very exciting, but it just means that the top of the pie was to be decorated. No cannibalism here.
This is the perfect pie to use up any leftover game meat, but you can also use, as it says in the recipe, rabbits and fowl from the fields.
The oldest version of this recipe can be found in an Anglonorman manuscript from the second quarter of the fourteenth century. There is another 'Teste de Turk' in an even older Anglonorman manuscript from 1290, but this a completely different recipe (pig's stomac stuffed with pork, chicken, saffron, eggs, bread and almonds, boiled. I guess the resulting giant meatball is the head). "Our" recipe is for a pie or pasty that is decoreated with the image of a head.
Anyway, the dish is meant as a subtlety, a dish to surprise and amuse guests. Compared to other subtleties like peacocks and swans in their feathers, or fire breathing boar heads this Teste de Tourk is rather tame. Another subtlety on this site is the Fake fish.
The illustration is a fragment of the Bayeus Tapestry depicting William the Conqueror's capture of England in 1066. Left to right: meat is roasted on skewers in a field kitchen while a pot is cooking over a fire. A baker is removing bread from a portable oven. Then the cooked dishes are being prepared to be served, and one servant blows a horn to announce dinner is ready. The guests are seated at the table. The red-haired man is William. Bishop Odon (to the right of William) speaks a blessing. A servant with a napkin over his left arm  carries another dish to the table.

The original recipe
I have chosen the version from the Anglonorman manuscript that was written circa1320-1340, British Library MS Royal 12.C.xii (edition). Anglonorman is the result of the dialect spoken by the noblemen from William the Conqueror from Normandy (a kind of French), and the Anglosaxon spoken by the conquered inhabitants (who were themselves victorious newcomers in the past). In this manuscript the language is much closer to French.
A translation of this text was part of a Middle English recipe collection De diuersa cibaria, in British Library Add. 46919 (edition in Curye on Inglysch). The recipe for Turk's Head has also been translated, and is named 'Teste de Turt', still very French sounding.

The modern adaptation of the recipe:
Printout version
I've allowed myself some freedom in interpretating this recipe. All modern adaptations agree that the Saracen's head is made with chopped and coloured pistachio nuts (like a mosaic), but I have chosen for a closed pie or pasty with decoration on top. Because I liked it I have also added some minced meat of pork or veal, but you can leave that out if you want to. The rabbit and fowl can be roasted or boiled, and if you have any leftover game (hare, boar, deer) you can use that too. The pistachio nuts are not coloured, because the decorative effect is not needed in a closed pie.
As to the decoration: nobody forces you to use a Turk's head. You can make a seasonal pasty with holly leaves, or put little rabbits and birds on it, or a cat devouring mice, whatever takes your fancy. Just as long as it is a spectacle to behold. Surprise and amuse your guests!

List of ingredients:
300 gram (2/3 pound) minced meat (pork or veal) (optional)
4 hindquarters of a wild rabbit (or one rabbit)
4 quails, or 2 partridges or pheasants
2 Tbsp. sugar
1/4 tsp. ground cloves
1 tsp. ground cubeb (or black pepper with a little piment)
200 gram (1 1/4 cup) dates
200 gram (3/4 cup) young, fresh cheese (sheep, goat, cow)
200 gram (1 1/2 cup) pistachio's without shells
60 gram (2 Tbsp. or 1 fl.oz) honey
lard, suet or butter
salt
dough for pasties
1 egg (optional)

Preparation in advance:
Fry the minced meat in lard, suet or butter.
Sprinkle rabbit and fowl with peper and salt. Heat lard, suet or butter in a large skillet, brown the meat quickly, then cover and simmer until it is done (about forty minutes). You can also roast the meat in the oven, baste regularly with the fat (suet, lard, butter). When it is done, let the meat cooluntil you can easily debone it. Cut into large chunks.
Steep the stoned dates five to ten minutes in honey that is heated with two tablespoons of water. Drain the dates, but keep the honeywater. Cut the dates in quarters.
Crumbe the cheese, or chop it.
Put everything in a bowl: minced meat, rabbit and fowl, spices, chees, dates, sugar and honeywater, mix well.
The crust: make a pasty dough, or use some ready-made if you really think you must. But making your own is more fun, and you get a special dough.

Preparation:
Heat the oven to 200ºC (400ºF).
Take a springform or a pie dish that is large enough to contain the stuffing (that depends on how large your rabbit and fowl were, whether or not you added minced meat, or how much leftovers you had). Grease the form with butter and roll out your dough. Place the dough in the piedish. If you use a springform, it is best to assemble the pasty: first cut out the bottom out of a rolled sheet of dough and place that in the springform. Then cut a long strip of dough, a little broader than the springform is high, and cover the sides. Be sure to seal the side to the bottom sheet of dough by gently pressing the edges togehter. If you want to be sure, roll a thin strip of dough between your palms and press that against the edges. Let the dough that hangs over the top of the form be, you'll use that to seal the cover.
Scoop the stuffing into the dough, cover with pistachio nuts. Close the pasty or pie with another sheet of dough. Press the edges of the cover and the sides together and cut out a small hole or two to let the steam escape. You can incorporate these holes into your decoration (eyes, mouth).
Now the name of the pasty becomes clear: use leftover dough to decorate the cover with a "Turk's head" or something else. Colouring and gilding is done after baking, but you can baste the dough with eggwhite (for a light glaze) or egg yolk (for a darker glaze).
Put the pasty or pie in the middle of the oven, bake for about forty minutes. Let cool five minutes after taking it from the oven befor demoulding.
To finish the decoration apply food colouring paste with a small brush, and gold leaf or silver leaf.

To serve:
A pasty like this one can be served hot as well as cooled to room temperature. Cut the cover loose and lift it, and scoop out the stuffing. When eating the medieval way, you use your fingers to pick what you want, and eat it above your bread trencher.

Ingredients
All descriptions of ingredients

Cubeb - One of the obsolete spices that is making a come-back because of today's wish for exotic dishes. The scientific name is Piper cubeba. It is also called Java pepper (three guesses why ...), or tailed pepper because of the stalks still attached to the berries. The taste is peppery and slightly bitter. As a substitute you can use black pepper with piment (allspice).
Gold leaf and silver leaf - You can eat gold and silver. They pass through your body without doing anything, either harmful of good (despite the claims of some therapies). So, why would you eat gold? Because it is expensive and glamorous of course!
Practically anything can be gilded. But first you must find your gold leaf or silver leaf! In the Netherlands one place to look is shops that supply artists. You can buy "booklets". The leafs are separated by tissue paper, and are carried by another thin sheet of paper, that you use to transfer the gold leaf, because it is so thin that they fall apart very easily. A booklet of 25 gold leafs of 8x8 centimeters cost about 25 Euro. Before you transfer the gold (or silver), make a 'glue' of one part egg white and four parts water, brush that on where you want the gold leaf to stick. If you want to gild on icing or frosting, only use egg white. Don't expect a shiny result like wedding ring, to get that, you'll have to use bolus (red earth) as base, and polish the gold afterwards.
Food to gild can be anything, like the rind of a melon, christmas cakes and other festive cakes and cookies, and of course this medieval pasty. In the Middle Ages gold leaf was also used to gild the beaks of large birds or the tusks of a boar's head. The "soufflé Rothschild", a recipe from Escoffier, is made with kirschwasser in which small pieces of gold leaf float.
Pheasants and partridges - These birds are related. Both are delicious fowl, much appreciated fare for the nobles since centuries. Not just because of their taste, partridge meat was excellent food to help recovering patients regaining their strength. Female partridges (hens) are tastier than males. That is because the females have a higher percentage of body fat (regrettably, the same goes for humans).
Pheasants are not indeginous to the British Isles. They were imported, first by the Romans, and again by the Norman conquerors in the eleventh century. 
The hunting season is from september to january. However, partridges are listed as a threatened or near threatened species in many countries and regions. So if you buy partridges, choose farm raised ones. They may taste 'less wild', but at least the wild birds are safe.
The hunting season for pheasants starts later in the year, half october to december (hens) or january (cocks). Often farm-raised pheasants are put out in the fields, sometimes even broken-winged. That is not hunting but target shooting. Better bring the birds straight to the poulterer's.
Rabbits - Sweet little animals, but they are not indigenous to the British Isles. They were introduced by the Normans, and of course, some escaped. Wild rabbits are smaller than domesticated once, and their meat is less white. In modern cookbooks you can find recipes for wild rabbit with recipes for hare and game, and recipes for domesticated rabbits in the poultry section.

Bibliography
The editions below are in my possession. Links refer to available editions. 
All books mentioned on this site

Constance B. Hieatt and Robin F. Jones, 'Two Anglo-Norman culinary collections edited from British Library manuscripts Additional 32085 and Royal 12.C.xii'. In: Speculum 61 (1986), pp.859-882.
Constance B. Hieatt en Sharon Butler, Curye on Inglysch. English culinary manuscripts of the fourteenth century (Including the 'Forme of Cury'). Oxford, 1985.

All text and pictures of dishes are the intellectual property of Coquinaria and may not be reproduced without permission and acknowledgement..