Slice beef sirloin or flank steak thinly against the grain
Season the beef with salt and black pepper
Heat oil in a large skillet or wok over medium-high heat
Sear the beef in batches until browned, then remove
Slice bell peppers and onion into strips
Add more oil if needed and cook the onion and peppers until slightly tender
Add minced garlic and cook briefly
Stir in soy sauce, beef broth, and a little cornstarch mixed with water
Add the beef back to the pan
Toss everything together until the sauce thickens and coats the beef and vegetables
Serve hot over rice or noodles