Put the lemon juice and about 2 tablespoons of olive oil into a roasting dish. Put in the lamb fillets sprinkled on both sides with black pepper and turn about in the mixture. Leave in a cool place, covered with a cloth or foil for about 8 hours, or overnight. Turn the fillets over whenever you remember and spoon some of the mixture over them. When marinated pour off the surplus mixture, sprinkle the fillets generously with powdered cumin. Fry very gently and slowly in a heavy pan, turning several times, for about 15–25 minutes. Transfer to a serving dish and keep warm. Slice the onions in rings as finely as possible. Fry them gently in the leftover fat and juices in the trying pan until transparent but not very soft. Arrange on top of the fillets. Heat the sour cream with any leftover juices in the frying pan just to boiling point. Pour over the fillets and serve. I think this dish is very good with new potatoes and a bean salad.