Step 1: Prepare the Chicken and Sauce

Begin by cutting the chicken breasts into bite-sized pieces, ensuring they are uniform in size for even cooking. In a medium-sized bowl, whisk together the oyster sauce, soy sauce, black pepper, cornstarch, salt, and water until the cornstarch is fully dissolved. This mixture will serve as the flavorful base for the sauce that will coat the chicken and vegetables. Set the chicken and sauce aside.

Step 2: Sauté the Chicken

Heat the vegetable oil in a large skillet or wok over medium-high heat. Once the oil is shimmering, add the chicken pieces to the pan in a single layer. Cook the chicken for about 5-7 minutes, or until it is golden brown on all sides and cooked through. Be careful not to overcrowd the pan, as this will lower the temperature of the oil and result in steamed chicken rather than seared chicken. If necessary, cook the chicken in batches. Once cooked, remove the chicken from the pan and set aside.

Step 3: Sauté the Vegetables

In the same skillet or wok, add the sliced onions and cook for 2-3 minutes, or until they become translucent and slightly softened. Next, add the minced garlic, sliced mushrooms, and chopped green bell pepper to the pan. Cook the vegetables for another 3-4 minutes, stirring occasionally, until they are softened and slightly browned. The mushrooms should release their moisture and become tender.

Step 4: Combine Chicken and Sauce

Return the cooked chicken to the skillet with the vegetables. Pour the prepared sauce over the chicken and vegetables, ensuring that everything is evenly coated. Stir well to combine.

Step 5: Thicken and Finish

Bring the sauce to a simmer and cook for 2-3 minutes, stirring constantly, until the sauce thickens and becomes glossy. The cornstarch in the sauce will help it to thicken quickly. Once the sauce has reached your desired consistency, remove the skillet from the heat and drizzle with sesame oil. Give it a final stir to incorporate the sesame oil, which will add a fragrant and nutty aroma to the dish.