In a small bowl, mix together the ingredients for the sauce – soy sauce, oyster sauce, rice vinegar, cornstarch, sugar, sesame oil, and chicken broth. Set aside.
Season the chicken pieces with salt and pepper.
Heat 1 tablespoon of vegetable oil in a large skillet or wok over high heat. Add the chicken and stir-fry until it is golden brown and cooked through, about 5 to 7 minutes. Remove the chicken from the skillet and set aside.
In the same skillet, add the remaining tablespoon of oil. Add the red bell pepper, green bell pepper, onion, and garlic. Stir-fry for 2 to 3 minutes until the vegetables are just tender but still crisp.
Return the chicken to the skillet with the vegetables.
Give the sauce a quick stir to recombine the cornstarch, then pour it over the chicken and vegetables. Stir to combine and cook for another minute or two until the sauce has thickened and everything is well coated.
Sprinkle in some red pepper flakes or sliced chili for an extra kick of heat. Garnish with sesame seeds and chopped green onions. Serve immediately, offering steamed rice or noodles on the side if desired.
