Prep Ingredients: Slice chicken into thin strips. Cut all vegetables into uniform pieces. Mince garlic.
Make Sauce: In a small bowl, mix soy sauce, oyster sauce, black pepper, red pepper flakes, and sesame oil. Set aside.
Cook Chicken: Heat skillet over medium-high heat. Add 1 tablespoon butter. Cook chicken strips 2-3 minutes per side until golden and cooked through. Remove to a plate.
Cook Aromatics: Add remaining butter to pan. Sauté onion for 2 minutes. Add garlic and cook 30 seconds.
Stir-Fry Vegetables: Add cabbage, carrot, and bell pepper. Stir-fry 4-5 minutes until tender-crisp.
Combine: Return chicken to pan. Pour sauce over everything. Toss well and cook 1-2 minutes until heated through.
Serve: Serve hot over rice or noodles. Garnish with sesame seeds if desired.