Slice the sirloin beef into thin strips against the grain. Marinate with 1 tablespoon soy sauce, 1 teaspoon cornstarch mixture, and a pinch of black pepper. Let sit for 5 minutes.

Cut bell peppers and onion into bite-sized pieces. Mince garlic and grate ginger.

Heat 2 tablespoons of vegetable oil in a large wok or skillet over high heat (around 200°C). Add marinated beef and cook until browned on all sides but still slightly pink inside (about 2–3 minutes). Remove from pan and set aside.

In the same wok, add remaining oil. Sauté garlic and ginger for 30 seconds until fragrant. Add sliced onions and cook until translucent. Then toss in bell peppers and sauté for another 2–3 minutes until they soften slightly.

Return the cooked beef to the wok. Stir in soy sauce, oyster sauce, hoisin sauce, and remaining cornstarch mixture. Toss everything together until the sauce thickens and coats the ingredients evenly.

Adjust seasoning with salt and additional black pepper if needed. Garnish with sesame seeds before serving hot over steamed rice or noodles.