In a bowl, toss the sliced steak with soy sauce and cornstarch. Let marinate for 15 minutes.
Heat 1 tbsp oil in a skillet over medium-high heat. Add steak and cook until browned. Remove and set aside.
Add the remaining oil to the skillet. Stir-fry the bell peppers and onion until slightly tender.
Add garlic and ginger, stirring for about 30 seconds.
Return the beef to the skillet. Add beef broth, oyster sauce, brown sugar, black pepper, and red pepper flakes.
Stir-fry for 2-3 minutes until the sauce thickens.
Serve hot over cooked white rice.