Step 1:

Start by marinating the steak. Thinly slice the flank steak across the grain, which will ensure the meat remains tender during cooking. In a bowl, combine the soy sauce, black pepper, and cornstarch. Toss the sliced steak in the mixture and let it sit for 10 minutes. The soy sauce will infuse the meat with flavor, while the cornstarch creates a protective coating to keep it juicy during cooking.

Step 2:

While the steak is marinating, prepare the vegetables. Slice the bell peppers into thin strips, keeping them uniform so they cook evenly. Slice the large onion into half-moons, and mince the garlic cloves. Set all the prepared vegetables aside.

Step 3:

Heat a wok or a large skillet over high heat. Add 1 tablespoon of oil and swirl to coat the bottom of the wok. Once the oil is hot and shimmering, add the marinated steak in batches (to avoid overcrowding). Stir-fry the steak for about 2-3 minutes until it’s browned and just cooked through. Remove the steak from the wok and set it aside on a plate.

Step 4:

In the same wok, add the remaining 1 tablespoon of oil. Add the minced garlic first and stir-fry for about 30 seconds until fragrant. Then, toss in the sliced onions and bell peppers. Stir-fry the vegetables for about 3-4 minutes until they begin to soften but still retain a bit of their crisp texture. You want the bell peppers to remain vibrant and slightly crunchy.

Step 5:

Return the browned steak to the wok with the vegetables. Pour in the oyster sauce, sugar, and water. Stir everything together to ensure the steak and vegetables are coated evenly with the sauce. Let the mixture simmer for another 2 minutes, allowing the sauce to thicken and coat the ingredients. Taste and adjust seasoning if necessary.