Slice the kidney finely and scald with hot water to which the vnegar has been added. Drain and dry.
Heat the olive oil in a small flameproof casserole. Add the onion, stir, then fry gently until soft and golden.
Add the kidney slices and turn in the hot oil. Sprinkle in the paprika and plenty of black pepper.
Pour in the sherry, increase the heat and bubble up. Stir in the breadcrumbs, garlic, parsley and bay leaf. Reduce the heat and simmer gently for 10 minutes.
Serve with chunks of rough bread and a green salad, accompanied by a glass of sherry.