Prepare the Chicken
In a bowl, toss the sliced chicken strips with cornstarch, kosher salt, and black pepper until each piece is evenly coated. This helps create a nice texture and locks in flavor, while the cornstarch will also help thicken the sauce later.
Cook the Chicken
Heat 1 tablespoon of vegetable oil in a large skillet over medium-high heat. Add the seasoned chicken and cook for 5-7 minutes, stirring occasionally, until browned on all sides and cooked through. The chicken should reach an internal temperature of 165°F (74°C). Transfer to a plate and set aside.
Sauté the Vegetables
Add the remaining tablespoon of oil to the same skillet. Toss in the chopped onion and sliced mushrooms, cooking for about 5 minutes while stirring occasionally until they're tender and lightly browned. The mushrooms will release moisture and then begin to caramelize, developing a deeper flavor.
Create the Sauce
Stir in the minced garlic, soy sauce, oyster sauce, sesame oil, and water. Mix everything together well and bring to a gentle simmer. The garlic should become fragrant but not burn.
Combine and Finish
Let the sauce simmer for 2-3 minutes until it thickens slightly. Return the cooked chicken to the skillet and mix everything together so the sauce coats all the ingredients evenly. The cornstarch from the chicken will help further thicken the sauce.
Season and Serve
Taste and add more freshly ground black pepper as desired. Garnish with sliced green onions if you like, and serve hot over rice or noodles.