Heat the oil in a 4-quart pot over medium heat. Add the red onion, cabbage, caraway seeds, and a little salt and pepper. (Make sure you use only a little salt; the sauerkraut that you’ll be adding later is salty.) Stir to coat the vegetables with the oil. Cover and cook for about 10 minutes, stirring occasionally until the cabbage and onions are wilted but still have a crunch. Stir in the mustard, mixing well. Add the sauerkraut and stir to combine. Cover and continue cooking for about 4 minutes, stirring occasionally, until heated through. Taste for seasonings.