Prepare the Sauce : In a small bowl, whisk together soy sauce, oyster sauce, rice vinegar, sugar, cornstarch, water, sesame oil, ginger, and minced garlic. Set aside.

Cook the Chicken : Heat 1 tablespoon of vegetable oil in a large skillet or wok over medium-high heat. Season the thinly sliced chicken with salt and black pepper, then add it to the hot skillet. Stir-fry for 4-5 minutes until the chicken is golden brown and fully cooked. Remove the chicken from the skillet and set aside.

Cook the Vegetables : In the same skillet, add the string beans, optional red bell pepper, and onion. Stir-fry for 2-3 minutes until the vegetables are slightly tender but still crisp. Add 2 tablespoons of water to the skillet, cover, and steam the vegetables for 2 minutes.

Combine Everything : Return the cooked chicken to the skillet. Pour the prepared sauce over the chicken and vegetables. Stir to coat everything evenly. Cook for an additional 1-2 minutes until the sauce thickens and everything is well combined.

Serve : Remove from heat and serve hot over steamed rice or noodles. Garnish with sesame seeds if desired.