1. In small bowl, stir first 3 ingredients until smooth. Place rib eye roast, fat side up, on small rack in large open roasting pan; spread top of roast with mustard mixture. Insert meat thermometer into center of roast. Roast meat in 350°F. oven about 1 hour 20 minutes or until meat thermometer reaches 140°F. for rare (about 20 minutes per pound), or until of desired doneness.
2. Meanwhile, in 4-quart saucepan over high heat, heat potatoes and enough water to cover to boiling. Reduce heat to medium-low; cover and cook 10 minutes; drain. Arrange potatoes around roast in drippings in pan, turning potatoes to coat them evenly with drippings. Bake 50 to 60 minutes along with roast, turning occasionally, until potatoes are fork-tender and browned. Remove meat to warm large platter; let meat stand about 15 minutes for easier carving. Keep potatoes warm.