Peel, wash and chop the sweet potatoes, cover and cook in salted water for about 15 minutes. In the meantime drain the sauerkraut in a sieve. Peel and finely chop the onions. Wash parsley, shake dry, pluck off leaves and chop finely.
Fry the bacon in a pan until crisp and remove. Fry the onions in the bacon fat for approx. 4 minutes until transparent. Add sauerkraut and apple juice and braise for about 10 minutes. Wash apples, quarter them, remove seeds, grate roughly and mix in. Season to taste with salt and pepper and braise for another 3-4 minutes. Stir in parsley.
Preheat oven (electric cooker: 200 °C/circulating air: 180 °C/gas: see manufacturer). Coarsely grate the cheese. Drain sweet potatoes and mash with a potato masher. Stir in 2 tablespoons of butter and season to taste with nutmeg.
Spread the sauerkraut and mashed sweet potatoes one after the other in four small or one large casserole dish. Break the bacon into large pieces, place on top and sprinkle with cheese. Bake in a hot oven for 15-18 minutes.
In the meantime clean and wash the spring onions and cut them into fine rings. Mix sour cream and milk. Season to taste with salt, pepper and paprika. Take the casseroles out of the oven, arrange with 1 blob of sour cream and spring onion rings.