Core the apples and cut into wedges (if you like, you can peel them beforehand). Peel and slice the onions.
Heat the oil in a saucepan or larger pan. Fry the onions in it until translucent. Add the apples and fry for about 5 minutes. Sprinkle the sugar on top and let it caramelize slightly. Deglaze with the apple juice. Pick the sauerkraut, add it and mix it carefully with the apples. Put the vegetable mixture in a baking dish. Cut the smoked salmon roast into 1 cm thick slices and spread on top of the sauerkraut.
Cook in a preheated oven at 180 ° C for about 30-40 minutes until the slices of Kassler are through. Serve hot.