1. Heat oven to 325F. Combine Walnut Crust ingredients; press evenly onto all surfaces of beef roasts.
2. Place roast on rack in shallow roasting pan. Insert ovenproof meat thermometer so tip is centered in thickest part of beef. Do not add water or cover. Roast in 325F oven 1 1/4 to 1 1/2 hours for medium rare doneness (Do not overcook.)
3. Remove roast when meat thermometer registers 135F. Transfer roast to carving board; tent loosely with aluminum foil. Let Stand 15 to 20 minutes. (Temperature will continue to rise about 10F to reach 145F for medium rare.)
4. Meanwhile combine mashed potatoes and cheese in large bowl; keep warm.
5. Carve beef roast into thin slices; season with salt and pepper as desired. Serve with mashed potatoes.