Peel and finely dice the onion and garlic. Heat the oil in a pot and sauté the diced onion and garlic. Add broth and lentils and simmer for about 15 minutes. In the meantime peel potatoes, carrots and turnip and cut into cubes.
Add vegetables and plums to the lentils and simmer for another 20 minutes. Season to taste with salt, pepper, caraway, vinegar and sugar. Wash the marjoram and dab dry. Pluck the leaves from the stalks and cut them finely.
Arrange the soup in plates and sprinkle with marjoram.