Simmer the lentils covered in 1 litre of water for about 30 minutes. In the meantime peel and wash the carrots and potatoes and cut them into small cubes. Peel and chop the onion. Cut sausage into fine slices.
Heat 1 tablespoon of oil. Sauté the onion in it. Add the lentils with the liquid, fill up with 1/2 litre stock, cook for another 10-15 minutes. Season to taste with salt, pepper, sugar and vinegar. Heat 1 tablespoon of oil.
Fry the diced vegetables for 1-2 minutes, season with salt and remove. Swivel the sausage slices in the frying fat and take them out. Deglaze roast with cream, boil down for 2 minutes at high heat. Season to taste with salt, pepper and mustard.
Arrange on plates.