In a medium bowl, whisk together black pepper, rice vinegar, light and dark soy sauces, brown sugar, toasted sesame oil, cornstarch, and water until smooth.

Cut the chicken breast into bite-sized cubes, marinate in a large mixing bowl with 1 tablespoon of the prepared sauce, salt, and cornstarch for about 10 minutes.

Heat peanut oil in a large skillet over medium-high heat. Add the marinated chicken and cook undisturbed for 4-5 minutes until golden brown, then flip and cook for another 3-4 minutes.

In the same skillet, add more oil if needed and stir-fry garlic, ginger, and chili pepper for about 30 seconds. Then add sliced onion and green bell pepper, cooking for 2-3 minutes.

Lower the heat, whisk the sauce again, pour over vegetables, and stir for about 1 minute until it thickens. Add chicken back, tossing to coat.

Remove from heat, sprinkle with freshly ground black pepper, and serve immediately with steamed or fried rice.