In a small bowl, whisk together soy sauce, oyster sauce, rice vinegar, cornstarch, sugar, sesame oil, and chicken broth until smooth; set the sauce aside.
Season the bite-sized 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 golden and cooked through, about 5–7 minutes; transfer the chicken to a plate and set aside.
Add the remaining 1 tablespoon of oil to the skillet, then add the sliced onion, red bell pepper, green bell pepper, and minced garlic.
Stir-fry the vegetables for 2–3 minutes until just tender but still crisp.
Return the cooked chicken to the skillet with the vegetables.
Give the prepared sauce a quick stir, pour it over the chicken and vegetables, and cook while stirring until the sauce thickens and coats everything, about 1–2 minutes.
Sprinkle with red pepper flakes to taste, then garnish with sesame seeds and chopped green onions before serving.