Heat a 12-inch skillet over medium heat, and add 1 tablespoon oil. Halve sausages lengthwise if desired. Cook until browned and heated through, about 3 minutes per side. Transfer to a plate, and cover to keep warm.
Add remaining tablespoon of oil to skillet if necessary. Add onion, and cook for 3 minutes. Add apples, stir, and cook until softened, 6 to 7 minutes. Stir in cider, and cook for 1 minute. Add sauerkraut, and heat until warmed through, about 2 minutes.
Transfer sauerkraut to a warm serving platter, and top with the sausages. Serve with pickles, cucumber, and mustards. (Sausages and sauerkraut can be kept warm, covered, in a 250-degree oven for up to an hour.)