In a bowl, combine 1 tablespoon of soy sauce, 1 teaspoon of sesame oil, and a pinch of black pepper. Cut your chicken breast into bite-sized pieces and add them to the bowl, ensuring they are well-coated. Let this mixture marinate for 10-15 minutes.

While the chicken marinates, whisk together the remaining soy sauce, oyster sauce (if using), black pepper, salt, and crushed red pepper flakes (if desired) in a small bowl. In a separate cup, mix cornstarch with a splash of water to create a slurry. Set both the sauce mixture and the slurry aside.

Heat a large skillet or wok over medium-high heat and add 1 tablespoon of butter until melted and bubbling. Add the marinated chicken to the skillet and stir-fry for about 4-5 minutes until it turns golden brown and is cooked through. Remove the chicken from the skillet and set it aside.

In the same skillet, add another teaspoon of butter if needed and toss in the chopped onion and minced garlic. Stir-fry these ingredients for about 2 minutes until they become fragrant.

Add the shredded cabbage, sliced bell pepper, and julienned carrot to the skillet. Stir them together and cook for 5-6 minutes to keep the vegetables tender-crisp.

Return the cooked chicken to the skillet, pouring the prepared sauce over everything. Stir well and cook for an additional 2-3 minutes until the sauce thickens.

Taste and adjust the seasoning with additional salt or pepper. Garnish with chopped green onions and serve hot over rice or noodles.