1. Preheat oven to 450ºF. Sprinkle pork with salt.
2. Warm oil in a large ovenproof skillet over medium-high heat. Add pork and brown on all sides, turning occasionally. Transfer to a plate and let cool slightly. Spread mustard over pork.
3. Add bacon to skillet and cook, stirring occasionally, until it begins to crisp, 1 minute. Add onion to skillet and sauté until tender, about 3 minutes. Add cabbage, vinegar and sugar and cook, stirring often, until cabbage has wilted, about 5 minutes.
4. Place tenderloin on top of cabbage and roast until a meat thermometer inserted into center of pork registers 150ºF, about 15 minutes. Let stand 5 minutes; slice pork and serve with cabbage.