In a bowl, toss the beef with soy sauce, oyster sauce, and cornstarch. Let it marinate for 15 minutes.
Heat vegetable oil in a pan over medium-high heat. Sear the beef for 2-3 minutes until browned. Remove from pan.
In the same pan, sauté garlic, ginger, and bell peppers for 2 minutes.
Return the beef to the pan. Add beef broth, hoisin sauce, and red pepper flakes.
Stir fry everything together for 2 minutes. Drizzle with sesame oil and toss.
Garnish with green onions and sesame seeds. Serve hot over rice or noodles.