Rognons au Porto

• 8-12 lamb kidneys
• 2 tablespoons butter
• ½ Spanish onion, finely chopped

Clean kidneys of fibres and fat; slice thinly. Melt butter in a frying pan; add finely chopped onion and parsley, and sauté, stirring constantly, for a few minutes. Season to taste with salt and freshly ground black pepper. Add kidneys and sliced mushrooms, and continue to cook, stirring continuously, until tender. Stir in flour; pour over port. Serve with croûtons fried in butter, or ri