Season the fish with pepper and salt and marinate with a little lemon juice. Mix together the cream, crème fraîche and paprika powder. Peel the onions, cut into rings and sauté in butter or margarine until translucent. Season to taste with pepper and salt.
Place the fish pieces side by side in a flat, lightly greased baking dish. Spread the onions on top, pour the cream and paprika mixture over it so that you can no longer see the fish. Bake in the convection oven without preheating for 45 minutes at 160 degrees.