Prime rib roast is the finest of beef roasts. It’s best if it cooks slowly, and it’s a real showpiece when brought to the table. Pull out that carving set!
Preheat oven to 275°F (140°C) and place roast upright in a roasting pan. Stir mustard, rosemary, salt and pepper and rub over surface of roast. Place uncovered in center rack of oven and cook for