Wash the kidneys under cold running water, drain and pat dry with absorbent paper. Peel off the thin membrane, if any, and snip out the central core with scissors. Cut the kidneys into thin slices.

Heat half the butter and half the oil in a large frying-pan. When sizzling fry the kidneys briskly for 2 - 3 minutes, stirring frequently. Remove from the pan and reserve.

Add the remaining butter and oil to the pan and, when hot, add the onion and saute very gently for 10 minutes.

Increase the heat a little, add the mushrooms and fry for another minute or so, stirring constantly.

Return the reserved kidneys to pan, add vermouth, cover and simmer for 3 minutes.

Finally stir in the soured cream and generous seasonings of salt and pepper and heat very gently, without boiling, for another minute.

Serve kidneys on a bed of rice, garnished with parsley and lemon wedges.