Start by cutting the boneless, skinless chicken thighs into bite-sized pieces in a large bowl. Coat the chicken evenly with cornstarch, ensuring each piece is well covered. Let the chicken sit for 15-30 minutes.

Heat about ½ inch of vegetable oil in a skillet over medium-high heat until shimmering. Add the chicken pieces in batches, frying for 5-7 minutes until golden brown and cooked through. Remove from skillet and set aside.

In the same skillet, lower the heat slightly and add the chopped green onions, minced garlic, and grated ginger. Sauté for 1-2 minutes until fragrant.

In a separate bowl, whisk together soy sauce, brown sugar, rice vinegar, sesame oil, and optional red pepper flakes. Pour this mixture into the skillet over the sautéed aromatics, bringing it to a simmer for about 1-2 minutes.

Return the cooked chicken to the skillet, stirring well to coat each piece evenly in the sauce. Allow to cook for an additional 2-3 minutes.

Plate your Mongolian Chicken and serve immediately with steamed white or fried rice.