Marinate the Chicken: Mix sliced chicken breasts with cornstarch, salt, and ½ teaspoon of freshly ground black pepper until evenly coated.
Cook the Chicken: In a large skillet, heat 1 tablespoon of vegetable oil over medium-high heat. Add marinated chicken slices and stir-fry until browned and cooked through, about 4-5 minutes. Remove from skillet and set aside.
Sauté Vegetables: In the same skillet, add another tablespoon of oil if needed, then toss in sliced onion and bell pepper. Stir-fry for about 2-3 minutes until softened, then add sliced mushrooms and minced garlic, cooking for another 2-3 minutes.
Prepare the Sauce: In a small bowl, whisk together soy sauce, oyster sauce, honey, rice vinegar, and an additional teaspoon of freshly ground black pepper until well combined.
Combine Ingredients: Pour the prepared sauce over the sautéed vegetables and add back the cooked chicken. Toss until well-coated and heated through for about 1-2 minutes.
Serve It Up: Garnish with chopped green onions and serve hot over steamed rice or noodles.