Heat cooking oil in a large skillet over medium-high heat.
Add the chicken strips and cook until browned on all sides. Remove from skillet and set aside.
In the same skillet, add garlic and onion. Sauté until fragrant and translucent.
Add the bell peppers and stir-fry for 3-4 minutes until slightly tender but still crisp.
Return the chicken to the skillet and mix well.
Add soy sauce, oyster sauce, and sesame oil. Mix thoroughly.
If using cornstarch, dissolve it in 2 tbsp of water and add to the skillet. Cook until the sauce thickens.
Season with salt and pepper to taste.
Serve hot over steamed rice or noodles.