Peel and finely chop the onion. Peel, wash and finely dice the potatoes. Wash the apple thoroughly, quarter it and cut out the core. Dice apple finely. Remove 1/4 of the apple cubes and sprinkle with lemon juice.
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Add the horseradish. Mash finely with a potato masher. Season soup with salt, pepper and sugar. Wash the marjoram, dab dry and pull the leaves off the stalks. Serve soup in soup bowls.
Serve sprinkled with remaining apple cubes and marjoram leaves.