Clean the Brussels sprouts, cut the stems crosswise and wash. Remove the outer leaves from some Brussels sprouts and cut larger heads in half. Cook in boiling salted water for about 8 minutes. Then drain, rinse with cold water and drain.
Peel, wash and chop the potatoes. Peel 2 apples, quarter them, remove seeds and cut them into small pieces. Cover the potatoes and cook in salted water for about 20 minutes.
Meanwhile, wash the lemon hot, dab dry and rub the peel. Squeeze half of it. Peel the onions. Dice 1 onion finely and cut the other into rings.
Heat 1 tablespoon of oil in a large frying pan. Fry the minced meat in it until crumbly. Season with salt and pepper. Add the whole Brussels sprouts and onion and fry for about 5 minutes, turning them over. Deglaze with cream and 1⁄4 l water.
Stir in the marjoram and lemon peel. Bring everything to the boil and simmer for 2-3 minutes. Season to taste with salt, pepper and 2-3 tbsp. lemon juice.
Wash, quarter, core and chop 1 apple.
Heat 1 tablespoon of oil in a second pan. Brown the onion rings in it. Fry the apple pieces briefly. Season with salt and pepper. Drain potatoes and apples. Add milk and butter and mash to puree.
Season to taste with salt and nutmeg. Spread the apple and onion mix over the puree and serve.