Peel the mushrooms and break them into pieces. If it be desired to serve them very white, throw the pieces, sprinkled with lemon juice, into boiling water; allow them to stand two or three minutes, then pass into cold water. Dry carefully and sauté in hot butter three or four minutes. To two cups of mushrooms add three tablespoonfuls of flour and half a teaspoonful of chopped parsley. Stir and cook until the flour has become absorbed by the butter, then add one cup of stock, milk or cream. Stir until the sauce boils, then simmer very gently ten minutes. Add one teaspoonful of lèmon juice, a dash of paprika, and one fourth teaspoonful of salt. Remove from the fire. Beat the yolk of an egg with two tablespoonfuls of milk or cream and stir into the mixture. Fill buttered shells with the preparation. Stir one cup of cracker crumbs into one third cup of melted butter, and spread the crumbs over the top of the preparation. Let the dish stand in the oven long enough to brown the crumbs.