03Heat butter in a skillet and add chopped leek, chopped onions, garlic, salt and pepper. Cook, stirring, until onions are wilted. Add tomatoes and cook, stirring often, about 2 minutes.04When tongue and lentils have cooked 15 minutes, add leek and onion in tomato sauce. Stir and continue cooking.05Continue cooking 30 minutes longer for a total cooking time of 45 minutes. Discard onion studded with cloves and bay leaf. Add parsley and stir. Serve tongue sliced with lentils on the side.