Meanwhile, in 10-inch nonstick skillet, heat oil over medium-high heat. Add beef; cook and stir 1 to 2 minutes until beef is no longer pink; remove from pan.
Add bell pepper and onion to skillet; cook and stir 1 minute. Add broccoli; cook and stir 1 minute longer. Stir in water and crushed red pepper. Reduce heat to low; cover and cook 3 to 4 minutes or until broccoli is crisp-tender.
In small cup, mix teriyaki sauce and cornstarch. Stir teriyaki sauce and beef into broccoli mixture in skillet. Cook and stir over high heat until sauce is thickened and heated through. Toss broccoli mixture with spaghetti. Garnish with green onions.