Boil the calves' feet in salted water until tender; then take out the bones. Fry 1 chopped onion in butter; stir in 1 tablespoonful of flour; add 1 cup of stock. Let boil with 1 bay-leaf, some parsley chopped fine and ¼ cup of vinegar, salt and pepper to taste. Then add the feet and let simmer ten minutes. Stir in the yolks of an egg and serve hot.