In a small bowl, whisk together the soy sauce, oyster sauce, brown sugar, sesame oil, cornstarch, and water until smooth.
Heat 1 tablespoon of vegetable oil in a large wok or skillet over high heat until shimmering.
Add the sliced beef in a single layer and sear for 2-3 minutes until browned; remove the beef from the pan and set aside.
Add the remaining tablespoon of oil to the pan, then add the bell peppers and onions. Stir-fry for 3-4 minutes until vegetables are crisp-tender.
Add the garlic and ginger to the vegetables and cook for 30 seconds until fragrant.
Return the beef to the pan and pour the prepared sauce over the mixture.
Toss continuously for 1-2 minutes until the sauce has thickened and coats the beef and vegetables evenly.
Serve immediately over steamed rice.