Clean and wash the leek and cut into thin rings. Peel, wash and roughly grate the carrot, celery and apple. Mix immediately with lemon juice.
Heat the oil in a pot. Sauté the leek and carrot mix for about 2 minutes. Add lentils and dried marjoram. Deglaze with 3⁄4 l water. Stir in stock. Bring to the boil and simmer for about 10 minutes.
Season soup with salt, pepper, cumin and balsamic vinegar. Wash marjoram, shake dry and pluck off leaves. Serve the soup with 1 dollop of sour cream. Serve sprinkled with fresh marjoram if desired.