Skin and clean a young woodchuck or muskrat and remove the kernels under the front legs and in the small of the back. Soak overnight in water seasoned with a handful of salt. In the morning wash, rinse, and wipe the animal dry. Put the meat in a large pan, cover with fresh water, and add a large bouquet garni composed of 6 sprigs of green celery leaves, 1 large bay leaf, and 2 sprigs of fresh thyme, all tied together, an onion stuck with a clove, 10 peppercorns, and salt to taste. Cook for about 2 hours, or until the meat is tender, skimming the broth from time to time. Remove the meat, discard the bouquet garni, and reduce the cooking liquid over a high flame to about 1 quart. In the reduced liquid cook until tender 2 onions, sliced, and 1 1/2 cups each raw potato cubes, chopped celery, and chopped carrots.
Remove the meat from the bones and cut it into small pieces. Put the meat in a deep baking dish and add the cooked vegetables. Strain the stock and reduce it to 1 generous cup. Add 3 tablespoons sherry and stir in 1/2 tablespoon flour mixed to a paste with 1 tablespoon butter. Pour the sauce over the meat in the baking dish. Top with 1 cup mushroom caps, cooked in butter and drained, and cover the top with flaky pie pastry. Moisten the edges and press them carefully around the rim of the dish. Cut several gashes in the pastry to allow for the escape of steam. Brush with melted butter and bake in a hot oven (425° F.) for 10 minutes, or until the crust is brown.