Of great delicacy and, in the past, much used as a garnish in combination with little quenelles for vols-au-vent and similar dishes, testicles have, apparently, fallen from grace—at least in the cookbooks; their abundance in the market is sufficient proof that they are still appreciated in French homes.
Whatever their ultimate preparation, they should always be first peeled, disgorged, and parboiled (usually they are soaked for several hours in cold water, then parboiled, before being peeled, which serves little purpose, the impermeable skins’ preventing disgorgement): Only the egg-like form of flesh is retained; it is contained within a tough, loose outer skin and two inner skins, the last of which clings tightly to the flesh. Split the skins the entire length of the swelled surface with a sharp knife—you will have to cut some quarter of an inch into the flesh—and carefully peel the oval flesh form free, damaging it as little as possible. Put to soak in a basin of cold water for about 3 hours, changing the water two or three times. Transfer to a saucepan, cover largely with cold water, salt, bring to a boil, simmer for 6 or 7 minutes, drain, plunge into cold water and leave until cool. They may be kept, drained and refrigerated, in a covered bowl or wrapped in plastic, for a day or so before being used.
Frivolities are often braised like sweetbreads, but prolonged cooking toughens the flesh. To best retain their tender delicacy, cut them into ¼-inch slices and stew them simply in butter, tossing occasionally, for 6 or 7 minutes, adding chopped fines herbes at the halfway point (tarragon is very much at home in this context) and finishing with a few drops of lemon juice. Cream may be added or a cream and egg-yolk mixture thickened over low heat, stirring constantly, without approaching the boil; they may be tossed in olive oil over high heat and finished with a persillade and a bit of lemon like the hearts and liver in the preceding recipe; or attractive stews may be imagined in combination with veal kidneys, sweetbreads, mushrooms, etc., each prepared separately to suit its demands and combined at the last moment in the sauce of one or another. Dipped in batter and deep fried, they are exquisite.