Cut chicken breast, onion, and bell pepper into bite-sized pieces.
Heat cooking oil in a pan and stir-fry the chicken breast.
When the chicken breast is halfway cooked, add onions and bell peppers, and stir-fry together.
Make a sauce by mixing gochujang (Korean chili paste), soy sauce, minced garlic, red pepper powder, and sugar.
Add the sauce to the stir-fried ingredients and quickly stir-fry over high heat.
Serve the stir-fried mixture over rice and garnish with chopped green onion and sesame seeds.