Marinate the chicken with salt, pepper, and oyster sauce, and let it sit overnight.

Heat the oil and sauté the minced garlic until golden brown.

Add the onion and sauté until it becomes soft.

Add the chicken, then add pepper, granulated sugar, sweet soy sauce, and sesame oil. Stir and cook until the sauce thickens and the chicken is cooked, about 15 minutes.

Add the bell peppers, stir briefly, then turn off the heat.