Drain and rinse with sauerkraut with cold water, then leave it to drain in a colander. Melt 4 tablespoons of the fat in a large flameproof casserole. Add the onions and saute them over low heat, stirring occasionally, until tender, 7 to 10 minutes. Spread half of the sauerkraut over the onions, first squeezing it in your fists to extract all the water. Set the apple, both kinds of bacon, the smoked sausages, and smoked pork loin to top. Sprinkle with juniper berries and add seasoning, mainly pepper. Cover the meat with the rest of the sauerkraut, squeezing it dry also, and pour over the wine. Cut a piece of parchment paper to fit inside the casserole and press it directly on top of the sauerkraut. Cover and simmer on top of the stove until the sauerkraut is tender, 1 1/2 to 2 hours. About halfway through cooking, pour over the schnapps.
Half an hour before the sauerkraut is cooked, put the potatoes in a pan of cold salted water, cover and bring them to a boil. Simmer until tender when pierced with a knife, 15 to 20 minutes. Drain and keep them warm, peeling them just before serving.
Heat 1 tablespoon fat in a frying pan and cook the cured pork chops until well done, 5 to 7 minutes on each side. Add them to the casserole 15 minutes before the sauerkraut is cooked. Warm the frankfurters in very hot water until heated through, about 1 minute. Leave them in the water to keep warm, draining them just before serving.
To serve, taste the sauerkraut and adjust the seasoning. Lift out both kinds of bacon and the pork loin and carve them in generous slices. Drain the sauerkraut well and pile it on a large heated platter. Arrange the meats and sausages on top, with the potatoes around the edge. Pass mustard at the table.