Melt 1 tablespoon butter in a large heavy skillet. Add roast; cook 4 minutes on each side or until browned. Remove roast from skillet, and let cool 10 minutes. Transfer roast to a rack in a lightly greased roasting pan.
Combine remaining 2 tablespoons butter, flour, and next 4 ingredients. Spread mustard mixture evenly over top and sides of roast. Insert meat thermometer into thickest part of roast, if desired. Place roast in a 450° oven. Reduce heat to 350°, and bake 1 hour and 20 minutes or until meat thermometer registers 160°.
Combine cranberry sauce and next 4 ingredients. Bring to a boil over medium heat, stirring frequently. Dissolve cornstarch in water; stir into cranberry mixture. Cook, stirring constantly, until mixture thickens.
Remove roast from pan, and let stand 10 minutes. Remove string from roast; cut diagonally across grain into 1/4-inch-thick slices. Serve with cranberry mixture.