Wash mushrooms; remove stems. Coat mushroom stems and caps with the flour; brown in butter about 5 minutes, until all moisture has evaporated. Add cream, sherry, salt and nutmeg; cook, uncovered, about 2 minutes, till heated through. Add a little of the hot mixture to beaten egg yolks; stir into mushroom mixture and cook, stirring constantly, till mixture bubbles. Cook 2 minutes more. Serve in warm patty shells.