In a mixing bowl, combine soy sauce, sesame oil, and black pepper. Add diced chicken and toss to coat. Marinate for 10-15 minutes.

In a separate bowl, whisk together remaining soy sauce, oyster sauce, black pepper, salt, and red pepper flakes. Prepare cornstarch slurry if desired.

Heat butter in a large skillet over medium-high heat. Add marinated chicken and stir-fry for 4-5 minutes until cooked through. Remove and keep warm.

In the same skillet, add onion and garlic. Sauté for 2 minutes until onion is translucent.

Add cabbage, bell pepper, and carrot. Stir-fry for 5-6 minutes until vegetables are crisp-tender.

Return chicken to skillet, pour in the sauce and stir-fry for another 2-3 minutes until sauce thickens.

Adjust seasoning as needed and garnish with sliced green onions before serving hot.