Peel and finely dice the onion and garlic. Heat the oil in a large pot. Fry the onion and garlic for about 5 minutes. Deglaze with the broth, add lentils and sausage and bring to the boil.
Cover and cook over medium heat for about 30 minutes. In the meantime clean, wash and cut the leek into pieces. Peel, wash and chop the potatoes and carrots. After about 15 minutes cooking time add leek, potatoes and carrots.
Take out the sausage and cut it into slices. Wash the marjoram, dab dry and chop finely. Add sausage and marjoram to the stew. Stir in vinegar. Season to taste with salt, pepper and sugar. Garnish lentil stew with marjoram.