1. In a small bowl, whisk together the soy sauce, oyster sauce, sesame oil, cornstarch, salt, and pepper. Set the sauce aside.

2. Heat 1 tablespoon of vegetable oil in a large skillet or wok over medium-high heat. Add the sliced chicken and stir-fry until cooked through and lightly browned, about 5-6 minutes. Remove the chicken from the pan and set it aside.

3. In the same skillet, add the remaining 1 tablespoon of vegetable oil. Add the onions and garlic, and stir-fry for 1-2 minutes until fragrant. Then, add the mushrooms and bell peppers, and continue to stir-fry for another 3-4 minutes until the vegetables are tender-crisp.

4. Return the cooked chicken to the skillet. Pour the prepared sauce over the chicken and vegetables, and stir well to coat everything evenly. Cook for another 2-3 minutes, or until the sauce has thickened and the dish is heated through.

5. Serve your delicious Chicken and Mushroom Stir-Fry hot over steamed rice or noodles.