In a medium bowl, mix the sliced beef with 1 tablespoon of soy sauce and cornstarch. Set it aside to marinate while you prepare the vegetables.
Heat the vegetable oil in a large skillet or wok over medium-high heat.
Add the marinated beef to the skillet and stir-fry for about 3-4 minutes until browned and just cooked through. Remove the beef from the skillet and set aside.
In the same skillet, add the sliced onion and cook for 2 minutes until slightly softened.
Add the red and green bell peppers, garlic, and ginger, and stir-fry for another 3-4 minutes until the peppers are tender-crisp.
Return the beef to the skillet, along with the beef broth, oyster sauce, sugar, and the remaining 1 tablespoon of soy sauce. Stir everything together and cook for an additional 2-3 minutes until well combined and heated through.
Season with salt and pepper to taste, then remove from heat.
Serve hot over cooked rice or noodles.