Preheat oven to 350°F. Place beef in roasting pan fat side up. Add approximately 1 cup of water to the bottom of the pan.
Combine mustard and brown sugar/honey. Spread half of the mixture over the top of the beef. Cover and seal roasting pan with aluminum foil.
Bake for 3 to 3 ½ hours. Remove roast and turn oven to broil. Remove foil and brush the beef with remaining glaze. Return to oven and broil for approximately 5 minutes until glaze turns golden.
Remove from oven, tent with foil and let rest 10 minutes. Slice against the grain.