Prepare the stir-fry sauce: In a small bowl, mix brown sugar, soy sauce, sesame oil, garlic, ginger and cornstarch.
Prepare the chicken: Wash the chicken and pat them dry with paper towels, then season with ground black pepper on both sides.
Saute pepper and onion: Heat the oil in a skillet over medium-high heat, add the bell peppers and onion and cook for 1-2 minutes until tender.
Transfer them into a bowl and set aside.
Cook the chicken: Turn the heat to high, add the chicken pieces in a single layer and cook for 4-5 minutes on each side until no longer pink.
Now, add the bell peppers and onion back to the skillet with the chicken, pour the stir-fry sauce over and simmer for 1-2 minutes until the sauce starts to thicken.
Garnish with sesame seeds and serve.
Notes
The soy sauce is quite salty, which is why I didn't mention adding any additional salt, but you can do a quick taste test and add if you feel necessary.