1. Preheat oven to 250 degrees Fahrenheit (120 degrees Celsius).
2. Heat oil in a heavy skillet (cast iron would be great). Salt the roast and sear on all sides. Remove to a baking dish. Brush with mustard and add rosemary, minced onion, and crushed garlic. Roast uncovered until internal temperature hits 120–130 degrees Fahrenheit (49–54 degrees Celsius), about one hour. Slice very thinly to serve.