Cook Brussels sprouts for minimum time as directed on box.
Meanwhile, in 12-inch nonstick skillet, heat oil over medium-high heat. Add beef; cook and stir 1 to 2 minutes or until beef is no longer pink; remove from pan.
Add bell pepper and onion to skillet; cook and stir 2 minutes. Add stir-fry sauce and red pepper flakes. Reduce heat; cook and stir about 2 minutes longer or until vegetables are crisp-tender. Stir in beef and Brussels sprouts & butter sauce; cook about 1 minute or until hot. Serve with rice.