Mix sliced chicken with cornstarch, salt, and ½ teaspoon black pepper until evenly coated.
Heat 1 tablespoon of oil in a large skillet over medium-high heat. Stir-fry the chicken until browned, about 4-5 minutes, then remove from skillet and set aside.
In the same skillet, add remaining oil and stir-fry onion and bell pepper for 2-3 minutes until they start to soften. Add sliced mushrooms and minced garlic; continue cooking for another 2-3 minutes until tender.
In a separate bowl, whisk together soy sauce, oyster sauce, honey, rice vinegar, and 1 teaspoon of freshly ground black pepper until combined well.
Pour the sauce over the sautéed vegetables and return the cooked chicken back into the skillet. Toss everything together until well-coated in sauce and heated through for about 1-2 minutes.
Garnish your dish with chopped green onions before serving hot over rice or noodles.