Cut the chicken breast into bite-sized pieces.
In a bowl, mix chicken breast, sugar, soy sauce, minced garlic, pepper, and sesame oil.
Heat cooking oil in a pan over medium heat and stir-fry the marinated chicken breast.
Once the chicken breast is cooked, add sliced bell peppers and onions and stir-fry together.
Taste and adjust seasoning, then transfer to a serving plate.