The sauce.

Purée the tuna, anchovies, capers, lemon peel, mustard, and garlic paste in the machine, then add the egg yolks and purée several seconds, until the mixture has thickened. Finally, with the motor running, start adding the oil in a very thin stream of droplets, and continue without pause until 3/4 cup of oil has gone in and the sauce has thickened into a heavy cream. It will not be as thick as mayonnaise, but should hold itself in creamy suspension - process in 1/2 cup or more of oil, depending on how thick a sauce you wish to have. Season carefully to taste with lemon juice, salt, and pepper - the sauce should have character, but should not be too strong in taste or it will kill off the turkey.

Slicing the turkey.

If you are using a poached turkey breast, discard the skin. Carefully remove in one piece the whole side of each breast from the carcass. Cut the meat at a slant crosswise (across the grain), into elegant slices less than 1/8 inch thick.

Assembling.

Spoon a layer of sauce in the bottom of a serving platter, and arrange the turkey on top, spreading each slice with a coating of sauce. Cover with plastic wrap and refrigerate for 24 to 48 hours before serving.

Serving.

Let the platter sit at room temperature for 20 minutes to take off the chill. Meanwhile sprinkle on the capers and parsley, and decorate with the lemon.