Bring 1/2 cup of water to a light boil in a small skillet. Add the oysters and poach for two minutes. Strain the pan contents, reserving the liquid. Return the liquid to a light boil and reduce by about a third.
Put the parsley, garlic, mushrooms, salt, pepper, and nutmeg into a food processor, and process into a very fine mixture. Add the butter and process into a coarse gritty puree.
Add the bread crumbs and the reserved stock from the oysters and process only long enough to mix everything thoroughly.
Place six oysters in the bottom of each of eight small gratin dishes. With a spatula, lightly pack down enough sauce to cover the oysters completely, filling the dishes nearly to the top. (You can prepare the dish to this point ahead of time and hold in the refrigerator. Take them out of the refrigerator a half-hour before the final baking.)
Bake the gratins of oysters in a preheated 400-degree oven until the tops are distinctly browned and crusty, and the sauce is heaving and steaming. Serve immediately with hot French bread for getting up the extra sauce.