1️⃣ Marinate the beef
Slice the sirloin thinly against the grain (at a slight angle if possible) before marinating for maximum tenderness. Toss sliced sirloin with oyster sauce, soy sauce, sake, sesame oil, cornstarch, and most of the black pepper. Rest for 15 minutes while you prep the vegetables.

2️⃣ Prep the vegetables
Slice the onion and peppers and mince the garlic. Having everything ready makes the stir-fry quick and stress-free.

3️⃣ Mix the sauce
Whisk together the soy sauce, oyster sauce, sake, sesame oil, black pepper and cornstarch.

4️⃣ Sear the beef
Heat half the oil in a skillet or wok over high heat. Cook the sirloin in batches until just seared and browned, then remove and set aside.

5️⃣ Sauté the veggies
Add the remaining oil to the pan. Stir-fry the onion, peppers, and garlic for 3–4 minutes, until tender-crisp.

6️⃣ Bring it together
Return beef to the pan, pour in any remaining sauce from the marinade, and add the remaining freshly cracked black pepper. Stir until glossy and cooked through.

7️⃣ Serve
Garnish with sliced scallions if desired (optional). Serve with jasmine or brown rice, quinoa, or cauliflower rice for a balanced, energy-boosting meal.