In a large bowl, combine the vegetable oil, brown sugar, soy sauce, minced garlic, rice vinegar, fresh ginger, and sesame oil. Whisk these marinade ingredients together.
Place the chicken cubes into the bowl of marinade, ensuring that they are thoroughly coated. Allow the chicken to marinate for at least 30 minutes, but for an even more taste, let it rest in the refrigerator overnight.
Thread the marinated chicken pieces onto the skewers with cubed bell peppers and onions if desired, ensuring an even distribution.
Preheat an outdoor grill or indoor grill pan to medium heat, gently place skewers on the grill, allowing them to sear and sizzle. Rotate the skewers occasionally, ensuring that each side is charred. Cook until the chicken is completely cooked or the internal temperature reached 165 ˚Degree.
Sprinkle sesame seeds to add a subtle nuttiness, and the chopped green onions.