Position a rack in the center of the oven and heat the oven to 450°F. Spread the mustard evenly over the pork and then sprinkle with the coriander, peppercorns, and salt, pressing so the spices adhere. Heat the oil in a 12-inch ovenproof skillet over medium-high heat. Cook the pork, turning it with tongs, until nicely browned on all sides, about 3 minutes per side. Put the skillet in the oven and roast until an instant-read thermometer in the thick end of the pork registers 155°F, 18 to 20 minutes. Let rest for at least 5 minutes before slicing thinly.