In a large bowl, combine the chicken, soy sauce, garlic, sesame oil, and black pepper. Mix well and marinate for at least 15 minutes.
In a separate bowl, whisk together the oyster sauce, soy sauce, rice vinegar, black pepper, water, and 1 tablespoon cornstarch. Set aside.
After marinating, toss the chicken pieces with the remaining 2 tablespoons cornstarch until evenly coated.
Heat 2 tablespoons vegetable oil in a large skillet or wok over high heat.
Add the chicken in a single layer and cook for about 3 minutes per side until golden brown and cooked through. Work in batches if necessary to avoid overcrowding. Remove the chicken and set aside.
Add the remaining tablespoon of oil to the pan.
Stir fry the onions, green bell pepper, and red bell pepper for 1 to 2 minutes until slightly tender but still crisp.
Add the garlic and ginger and cook for another minute until fragrant.
Return the chicken to the pan and pour in the sauce mixture.
Toss everything together and simmer for 1 to 2 minutes until the sauce thickens and coats the chicken beautifully.
Serve the Easy Pepper Chicken Stir Fry hot with steamed rice, noodles, or sautéed vegetables.