In small bowl, combine brown sugar, red pepper flakes, bourbon, soy sauce, oil and mustard; beat with wire whisk until sugar has dissolved. Place beef in shallow glass baking dish. Pour bourbon mixture over beef; toss to coat thoroughly. Let stand at room temperature for 15 minutes to marinate.
Heat grill. When ready to grill, remove beef from marinade; reserve marinade. Alternately thread beef, bell pepper and onion onto four 12-inch metal skewers. Place kabobs on gas grill over medium-high heat or on charcoal grill 4 to 6 inches from medium-high coals. Cook 7 to 9 minutes or until beef is of desired doneness, turning once.
Meanwhile, place marinade in small saucepan. Bring to a boil. Boil until sauce measures 1/2 cup, stirring occasionally. Remove from heat; let stand 2 minutes to thicken. Spoon glaze over kabobs.