Peel and finely dice the onions. Peel, wash and dice the potatoes. Clean, wash and cut leek into rings. Wash marjoram, dab dry and pluck the leaves from stems.
Heat the oil in a casserole dish, fry the onions until translucent, add the brown lentils and 2/3 of the marjoram leaves, deglaze with vegetable stock and cook for about 45 minutes. Season with salt, pepper and sugar.
In the meantime, halve the apricots and cut the sausage into slices. After about 25 minutes of cooking time, add potatoes, apricots, leek, red lentils and sausage to the pot and cook until done.
Season stew again with salt, pepper, sugar and vinegar. Serve the stew sprinkled with the remaining marjoram.