In a small bowl, prepare the stir-fry sauce by combining brown sugar, soy sauce, sesame oil, garlic, ginger, and cornstarch.
After washing and patting the chicken dry with paper towels, season it with ground black pepper on both sides.
Heat 1 tbsp of olive oil in a skillet over medium-high heat. Add the chopped bell peppers and sliced onions, sautéing for 1-2 minutes until they are tender but still crisp. Transfer the vegetables to a bowl and set aside.
Increase the heat to high, add the remaining olive oil, and cook the chicken in a single layer for 4-5 minutes on each side until it is no longer pink.
Return the sautéed bell peppers and onions to the skillet with the chicken. Pour the stir-fry sauce over the mixture and simmer for 1-2 minutes until the sauce thickens, stirring occasionally to mix everything well.
Garnish with sesame seeds before serving.
Notes
For added flavor, consider adding a sprinkle of red pepper flakes for some heat.