Coat all of the 1 1/2 pounds boneless skinless chicken thighpieces in the 1/4 cup cornstarch.
Using a large sauté pan over medium high to high heat, add 1/4 cup avocado oil. When oil is extremely hot, add chicken pieces with excess corn starch shaken off to pan in a single layer. If needed, you can cook in batches. Allow to cook until golden brown on bottom, then flip to cook other side. Once both sides are golden brown, you can shake pan to loosen and distribute oil just to make sure all sides are crisp.
When chicken is done, transfer to a separate plate and discard excess oil, leaving a thin layer to coat the bottom of the pan.
Make the sauce:
While the chicken is cooking, combine all of the sauce ingredients in a bowl and whisk together.
3 tablespoons rice vinegar, 3 tablespoons soy sauce, 2 tablespoons sugar, 1 tablespoon hoisin sauce, 1 tablespoon cornstarch, 2 teaspoons ginger, 2 cloves garlic, 1/2 teaspoon red chili flakes, 1/4 cup water
Using the pan you cooked the chicken in over medium heat, add the sauce and stir while scraping the bottom of the pan to release anything that may have stuck to the bottom of the pan. Sauce will thicken in just a couple minutes.
Combine sauce and chicken:
As soon as the sauce thickens, reduce the heat to low, add the cooked chicken, and gently toss to coat in the sauce.
Serve immediately with sliced green onions and sesame seeds as garnish, if desired.
Notes
Do not over crowd: The amount of chicken I made barely fit in the pan. You want to ensure that each piece of chicken is touching the bottom of the pan in order for it to brown properly.
Double the recipe: This recipe is good for a hungry family of four, but can easily be doubled. Just double or triple the recipe, cook the chicken in batches, and once it's all done you can toss it in the sauce.
Leftovers and reheating: Doubtful you'll have any leftovers, but if you do, store them in an air tight container in the refrigerator. Reheat in the microwave at 50% power in one minute increments, stirring after each minute, until heated through.