Slice the beef. Freeze the beef for 15–20 minutes to firm it up. Slice thinly against the grain into bite-size strips. Pat dry with paper towels.

Marinate quickly. In a bowl, mix 2 tablespoons soy sauce, 1 teaspoon cornstarch, and 1 teaspoon oil. Add the beef and toss to coat. Let sit 10–15 minutes while you prep vegetables.

Make the sauce. In a small bowl, whisk 3 tablespoons soy sauce, 1–2 tablespoons oyster sauce, 1 tablespoon rice vinegar, 1–2 teaspoons brown sugar or honey, 1 teaspoon cornstarch, and a splash of water (2–3 tablespoons). Adjust sweetness and salt to taste. Add chili if you like heat.

Prep vegetables. Slice bell peppers and onion. Mince garlic and grate ginger. Keep everything near the stove for a quick cook.

Heat the pan. Set a large skillet or wok over high heat. Add 1 tablespoon oil and let it shimmer. The pan should be very hot before the beef goes in.

Stir-fry the beef. Add the beef in a single layer. Don’t crowd; work in batches if needed. Sear 1–2 minutes per side until browned but still slightly pink. Transfer to a plate.

Cook the aromatics. Add another teaspoon of oil if the pan looks dry. Toss in garlic and ginger. Stir for 15–20 seconds until fragrant, but don’t let it burn.

Stir-fry the vegetables. Add bell peppers and onion. Cook 2–3 minutes, tossing often, until crisp-tender and lightly charred in spots.

Combine. Return beef and any juices to the pan. Give the sauce a quick stir (cornstarch settles) and pour it in. Toss everything for 1–2 minutes until the sauce thickens and coats the beef and peppers.

Finish. Turn off heat. Drizzle with sesame oil if using. Top with sliced green onions and sesame seeds.

Serve. Spoon over warm rice or toss with noodles. Grind a little black pepper on top and enjoy.