1. Thoroughly wash green onions. Remove root ends and tips of onions; cut into 2-inch pieces. Place pieces in a large bowl.
2. Cut steak across the grain into thin slices, and add to green onions in bowl. Sprinkle with 2 Tbsp. cornstarch and soy sauce, tossing to coat.
3. Sauté beef mixture and red bell pepper, in 2 batches, in hot vegetable oil and, if desired, sesame oil in a large nonstick skillet over medium-high heat 3 minutes or until beef is done and bell pepper is crisp-tender. Whisk together beef broth, vinegar, red pepper flakes, and remaining 1 Tbsp. cornstarch. Add to beef mixture in skillet, and bring to a boil. Cook 1 minute or until thickened. Top with peanuts, if desired.
*1 lb. skinned and boned chicken breasts may be substituted.