Mix everything together in a small bowl except the beef, salt and pepper, and onion. It will have the consistency of a very loose paste. Set aside. Pat the roast dry with paper towels. With a meat fork, poke holes all over the roast. Put the roast in a zip-lock bag that is large enough to hold it. Pour the mustard mixture over the roast and massage it into the meat. Close the bag removing as much air as possible. Allow the roast to marinate in the refrigerator for at least two hours, even over night. An hour before you want to begin cooking the roast remove from the refrigerator and allow the roast to warm to room temp.
Preheat the oven to 325 F. On the stove top heat a large cast iron skillet. Remove the roast from the bag. Sear the roast on all sides for about 2 minutes per side. Place the roast in a large roasting pan, fat side up, on a bed of sliced onion. When the oven comes to temperature, place the roast in the oven, uncovered. If using a temperature probe, set the timer for your desired doneness. Or you can cook for about 25 to 30 minutes per pound for a rare to medium rare roast.