In a medium-sized bowl, mix the sliced beef with a pinch of black pepper and a tablespoon of soy sauce. Marinate it for around 15 minutes.
In a large skillet, heat a tablespoon of vegetable oil over medium-high flame.
Add the beef and stir-fry until it is cooked through, it may take around 4-5 minutes. Take the beef out from the skillet and put it aside.
In the similar skillet, add the remaining one tablespoon of vegetable oil.
Add minced garlic and sliced onion. Stir-fry until the onion is fragrant and translucent, it may take around 2-3 minutes.
Put chopped tomatoes and bell peppers (if you are using). Cook until the tomatoes are softened, it may take around 3-4 minutes.
Return the cooked beef to the skillet. Include the remaining one tablespoon of oyster sauce, soy sauce and sugar. Mix everything well.
Add the cornstarch mix and mix until the sauce turns thick, it will take around 1-2 minutes.
Before serving, let’s taste and adjust the seasoning using salt and pepper.
You can garnish it with green onions or fresh cilantro.
It’s time to serve the dish. You can serve it with cooked rice.