Clean and wash the leek and cut into rings. Peel and wash the carrots, halve lengthways and cut into slices. Peel onion and chop finely.
Heat butter or margarine in a large saucepan and sauté the vegetables in it for about 5 minutes. Add about 1200 ml of water and the lentils to the vegetables. Bring to the boil and stir in the broth. Simmer gently for about 8 minutes. Slice the sausages and add them to the soup. Simmer until the lentils and vegetables are soft. Season to taste with salt, pepper, a pinch of sugar and the fruit vinegar. Sprinkle with chopped parsley before serving.