Step 1: Prepare the Chicken
Start by cutting the boneless, skinless chicken breasts into bite-sized pieces. In a small bowl, mix the cornstarch with a pinch of salt and pepper. Coat the chicken pieces in the cornstarch mixture, making sure each piece is evenly covered.

Step 2: Prepare the String Beans
Wash and trim the ends off the string beans. You can break them in half if desired to make them easier to eat. Blanch the string beans by placing them in a pot of boiling water for about 2 minutes until they turn bright green, then quickly transfer them to a bowl of ice water to stop the cooking process. This keeps them crisp and vibrant.

Step 3: Make the Sauce
In a small bowl, combine the soy sauce, oyster sauce, chicken broth, brown sugar, rice vinegar, and red pepper flakes (if using). Stir until the sugar is dissolved and everything is well mixed. Set the sauce aside.

Step 4: Stir-Fry the Chicken
Heat 1 tablespoon of vegetable oil and 1 tablespoon of sesame oil in a large skillet or wok over medium-high heat. Once the oil is hot, add the coated chicken pieces. Cook the chicken for about 5-7 minutes, turning occasionally, until it’s golden brown and cooked through. Remove the chicken from the pan and set it aside.