Preheat oven to 475 degrees. Place pork in a 9-by-13-inch roasting pan. Rub pork with oil; evenly coat with mixed seeds. Season with 1 teaspoon salt and 1/2 teaspoon pepper.
Roast pork until browned, 25 to 30 minutes. Reduce oven temperature to 350 degrees. Continue cooking (tented with foil if browning too quickly) until an instant-read thermometer inserted into thickest part registers 140 degrees and pork juices run clear, 10 to 20 minutes.
Transfer pork to a platter. Tent with foil, and let rest 10 to 20 minutes while you make the Balsamic Gravy. To serve, slice the pork about 1/4 inch thick, and serve with the gravy.