Marinate Chicken:
Combine diced chicken with soy sauce, sesame oil, and black pepper; let sit for 10–15 minutes to marinate.Mix Sauce:
During marination, in a separate bowl mix together the rest of the soy sauce, oyster sauce (if using), black pepper, salt, crushed red pepper flakes (if using), and the cornstarch-water slurry.Sear Chicken:
Heat a large skillet or wok over medium-high heat, add butter, and cook the marinated chicken until golden brown on all sides.Sauté Aromatics:
Take out the chicken; in the same pan, sauté sliced onion and minced garlic until aromatic.Stir-fry Vegetables:
Toss in chopped cabbage, sliced bell pepper, and julienned carrot; stir-fry until vegetables are crisp-tender.Combine and Thicken:
Put the chicken back into the skillet, pour the prepared sauce over everything, and cook until the sauce thickens and the dish is hot.