In a medium saucepan over low to medium heat, add 2 tablespoons of vegetable oil. Once heated, add the garlic and ginger. Cook and stir for about 30 seconds until fragrant.
Next, whisk in the dark brown sugar, red pepper flakes, soy sauce, rice vinegar, and water. Allow the mixture to cook until slightly thickened, which should take about 5 to 10 minutes. Set this sauce aside for later use.
In a large bowl, add the thinly sliced chicken and sprinkle cornstarch over it. Mix well until the cornstarch evenly coats the chicken pieces.
Heat 1 cup of vegetable oil in a large saucepan. Once hot, carefully add the coated chicken pieces. Fry them until they are browned and cooked through, which should take about 1 to 2 minutes. Stir the meat gently to ensure even cooking.
Once cooked, transfer the fried chicken to a paper towel-lined plate to absorb any excess oil. Discard the used oil from the skillet to keep the dish light.
Return the cooked chicken to the skillet. Pour the reserved sauce over the top. Cook over medium heat for about 2 to 3 minutes until the sauce thickens and coats the chicken beautifully.
Stir in the sliced green onions and let them cook for an additional minute. If desired, garnish with sesame seeds for added texture.
Serve the Mongolian Chicken immediately over rice, allowing the sauce to soak in. Enjoy the burst of flavors in every bite!