Marinate beef with soy sauce, garlic, ginger, and sesame oil for 15–30 minutes.
Heat oil in a hot skillet. Sear beef in batches until browned, remove and set aside.
In the same pan, stir-fry onions and peppers until slightly tender.
Return beef to pan and add oyster sauce. Toss everything together.
Serve hot over freshly steamed jasmine rice.
Top with green onions and sesame seeds if desired.
