Ingredients

For the Marinade:
1 pound beef sirloin or flank steak, thinly sliced against the grain
2 tablespoons soy sauce
1 tablespoon oyster sauce
1 teaspoon cornstarch
1 teaspoon sesame oil

For the Stir-Fry:
2 tablespoons vegetable oil
1 large onion, sliced
1 red bell pepper, sliced
1 green bell pepper, sliced
2 cloves garlic, minced
1 teaspoon fresh ginger, grated

For the Sauce:
1/4 cup soy sauce
2 tablespoons hoisin sauce (optional)
1 tablespoon cornstarch mixed with 2 tablespoons water (slurry for thickening)
1/2 teaspoon black pepper (or more, to taste)
1/4 teaspoon red pepper flakes (optional, for spice)

Directions

In a bowl, mix the soy sauce, oyster sauce, cornstarch, and sesame oil.

Add the beef slices to the marinade and toss to coat evenly. Let sit for at least 15 minutes (up to 30 minutes for deeper flavor).

In a small bowl, whisk together the soy sauce, hoisin sauce (if using), black pepper, and the cornstarch slurry. Set aside.

Heat 1 tablespoon of vegetable oil in a large skillet or wok over high heat. Add the onion and bell peppers. Stir-fry for 2–3 minutes, until slightly softened but still crisp. Remove and set aside.

Add the remaining 1 tablespoon of oil to the skillet. Add the marinated beef in a single layer and sear for 1–2 minutes per side until browned but not fully cooked. Add the garlic and ginger, stirring for 30 seconds until fragrant.

Return the cooked vegetables to the skillet with the beef. Pour in the prepared sauce and toss everything together. Stir-fry for another 2–3 minutes, or until the sauce thickens and coats the beef and vegetables evenly.

Serve hot over steamed rice or noodles.

Garnish with sliced green onions or sesame seeds, if desired.