How to Make It
In a small bowl, stir the ketchup with the soy sauce, honey, vinegar and mustard until combined.
Preheat the oven to 375°. In a large ovenproof skillet, heat the oil until shimmering. Season the pork loin with salt and pepper and sear it in the skillet over moderately high heat until browned all over, about 6 minutes. Transfer the skillet to the oven and roast the pork for 30 minutes. Slather the pork with some of the barbecue sauce and roast for about 10 to 15 minutes longer, or until an instant-read thermometer inserted in the center of the roast registers 140°. Transfer the pork loin to a cutting board and let rest for 10 minutes.
Add the remaining barbecue sauce to the skillet and cook over moderately high heat, scraping up any browned bits, until the sauce is slightly caramelized, 5 to 6 minutes. Thinly slice the pork and serve the warm barbecue sauce alongside.