Peel and wash the potatoes and cook in salted water for about 25 minutes. Peel the onion. Dice 1/2 onion finely, cut the rest into 4 rings. Wash, clean and chop the peppers. Heat 2 tablespoons of oil in a saucepan, fry diced onion and bell pepper for about 3 minutes until translucent.
Add the sauerkraut, stock, bay leaf and allspice and simmer in a closed saucepan for about 30 minutes. In the meantime cut the cucumbers lengthwise into 6 thin slices. Wash the meat, dab dry and pat thinly between 2 layers of foil.
Spread with 1/2 tsp. mustard on each. Cover each with 1 slice of bacon, 3 slices of cucumber, 1 onion ring and 2 sage leaves, fold over and pin each with a wooden skewer. Heat 2 tablespoons of oil in a large frying pan and fry the Saltimbocca for about 4 minutes on each side.
Season with salt and pepper. Bring milk and butter to the boil. Drain potatoes, add hot milk. Mash everything with a potato masher. Season the puree with salt, pepper and nutmeg. Season sauerkraut with salt, pepper and sugar and arrange on plates with Saltimbocca.
Add the purée.