Melt the butter in the top of a double boiler, add the flour, mix carefully, and pour in the stock slowly. Add the vhestnuts, parsley, mushrooms, and seasonings. When the sauce has thickened, beat together the eggs and the cream, pour the sauce into the egg mixture, and return to the double boiler. Stir constantly until thick, mix in the sweetbreads, and pour into a buttered casserole. Mix the breadcrumbs and cheese and sprinkle over the casserole. Bake in a 140ºC oven for 15 to 20 minutes. To Prepare Sweetbreads: Soak in cold water for 25 to 30 minutes. Remove the membranes and simmer in water for 15 minutes to each quart of water add 2 thin slices of lemon and 1 tsp of salt. Cool and use in any recipe calling for sweetbreads. These precooked sweetbreads may be prepared in quantity and frozen.