Combine first 5 ingredients; rub on all surfaces of roast. Place roast on a rack in a roasting pan.
Bake, uncovered, at 450° for 20 minutes. Reduce heat to 300°, and bake 2 hours and 15 minutes or until a meat thermometer inserted in thickest portion registers 145° (medium rare) or 160° (medium). Let stand 10 minutes before slicing. Transfer roast to a serving platter, reserving 1/4 cup drippings for Blue Cheese Yorkshire Puddings; keep warm. Serve with Horseradish Cream. Garnish, if desired.
English mustard is an extremely hot condiment containing ground mustard seeds and turmeric. It contributes high flavor to the coating that bakes on the prime rib.