Parboil and drain oysters. Reserve liquor, strain, and set aside for sauce. Cook onion and mushroom in butter five minutes, add flour, and pour on gradually oyster liquor and chicken stock. Season with salt, pepper, and cayenne. Remove tough muscles from oysters, and discard. Shape oysters, cover with sauce, and coll on a plate covered with stale bread crumbs. Dip in egg and stale bread crumbs, fry in deep fat, and drain on brown paper.