Prep the Chicken: Slice the chicken into thin strips and toss with soy sauce, black pepper, and cornstarch. Let it marinate while you prepare the other ingredients.
Sear the Chicken: Heat oil in a large skillet over medium-high heat. Add the chicken in a single layer and cook until browned and cooked through. Remove from the pan and set aside.
Stir-Fry the Veggies: In the same skillet, add a bit more oil if needed, then sauté the bell peppers, onions, garlic, and ginger until just tender.
Make the Sauce: In a small bowl, mix together soy sauce, oyster sauce, rice vinegar, additional black pepper, and a splash of water. Pour into the skillet and bring to a simmer.
Combine and Finish: Add the cooked chicken back to the pan and toss everything together until the sauce is thick and glossy.
Serve hot with rice, noodles, or on its own for a low-carb option.