Trim fat from roast. Place roast in a shallow dish. Combine wine and next 6 ingredients, stirring well. Pour mixture over roast. Cover and marinate in refrigerator 8 to 10 hours, turning occasionally.
Remove roast from marinade, reserving marinade. Place roast on rack of a broiler pan coated with cooking spray. Insert meat thermometer into thickest part of roast, if desired. Pour water into broiler pan. Cover with aluminum foil, and bake at 450° for 20 minutes. Uncover and bake an additional 55 minutes or until meat thermometer registers 150° (medium-rare), basting roast frequently with marinade.
Let roast stand 15 minutes; cut diagonally across grain into thin slices. Transfer sliced meat to a serving platter, and keep warm.
Coat a Dutch oven with cooking spray; place over medium-high heat until hot. Add onion; cook 15 minutes or until tender, stirring frequently. Combine milk, cornstarch, and mustard, stirring until smooth. Gradually add milk mixture to onion, stirring constantly until thickened and bubbly. (Mixture will first appear curdled but will smooth out as it cooks.) Serve with roast.