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Peel the potatoes and divide them into chunks. Add them to a large pan with water and salt and bring to a boil. Cook the potatoes until they're completely done and meanwhile peel the apples, remove the core and chop them into not too small pieces.
Drain the sauerkraut if needed and set aside. Rinse sage leaves and chop them not too fine.
Cook the bacon until they're a bit crunchy and then also add the sauerkraut. Stir well and add it to the bottom of an oven dish. Pre-heat the oven to 200 degrees Celsius or 390 degrees Fahrenheit.
Drain the potatoes as soon as they're done and mash them using a potato masher. This doesn't have to be very smooth, then stir in a few cubes of butter and a dash of milk.
Add pieces of apple and sage and season with pepper and salt. Divide the mashed potatoes over the oven dish. Finish with a layer of bread crumbs and a some small cubes of butter over the top.
Bake the casserole for about 20 minutes in the oven, or until it's beautiful golden brown. Garnish with some more sage if you like, enjoy!