Soak 4 (12-inch) bamboo skewers in water 20 minutes.
Place shrimp in large plastic food bag. Combine soy sauce, oil, garlic and crushed red pepper in cup; mix well. Pour over shrimp. Close bag securely; turn to coat. Marinate at room temperature 10 to 15 minutes.
Drain shrimp; reserve marinade. Alternately thread shrimp and onions onto skewers. Place on rack of broiler pan. Brush with reserved marinade; discard remaining marinade.
Broil shrimp, 5 to 6 inches from heat, 5 minutes. Turn shrimp over; broil an additional 5 minutes or until shrimp are opaque.