Mix sliced chicken with cornstarch, salt, and ½ teaspoon black pepper until evenly coated.
Heat 1 tablespoon oil in large skillet over medium-high heat. Stir-fry chicken until browned, 4-5 minutes. Remove and set aside.
In same skillet with remaining oil, stir-fry onion and bell pepper for 2-3 minutes. Add mushrooms and garlic, cook another 2-3 minutes.
Whisk together soy sauce, oyster sauce, honey, rice vinegar, and 1 teaspoon black pepper.
Add sauce to vegetables, return chicken to skillet. Toss until well-coated and heated through, 1-2 minutes.
Garnish with green onions and serve hot over rice or noodles.