Preheat the oven to 475 degrees F. Place the beef in a roasting pan, fat-side up, and rub with the ground mustard, pepper and sea salt. Transfer to the oven to brown for 15 minutes. Lower the oven temperature to 325 degrees F and cook for 1 hour, until the roast begins to drip. Add the pearl onions, mushrooms and potatoes to the roasting pan and continue to cook until a meat thermometer inserted into the thickest part of the beef registers 135 to 140 degrees F. Transfer to a cutting board to rest. Slice the beef and serve on a large platter with the roasted onions, mushrooms and potatoes.