Begin by cutting boneless skinless chicken thighs into 1-inch cubes. In a mixing bowl, create a batter by combining all-purpose flour, cornstarch, salt, black pepper, one beaten egg, water, and a splash of vegetable oil. Coat the chicken pieces thoroughly and let them marinate in the fridge for at least 30 minutes.

Heat vegetable oil in a deep skillet or frying pan over medium-high heat until it reaches 350°F (177°C). Carefully drop the marinated chicken pieces in batches to avoid overcrowding the pan. Fry for about 2-3 minutes or until each piece turns golden brown and crispy.

In a medium saucepan, combine fresh orange juice and zest, soy sauce, rice vinegar, brown sugar, and minced garlic. Stir the mixture over medium heat until it begins to boil, allowing the flavors to meld together. This should take around 5 minutes.

Prepare a cornstarch slurry by mixing cornstarch with a little water in a separate bowl until smooth. Once the sauce is boiling, reduce the heat and gradually stir the slurry into the sauce, stirring for about 1-2 minutes until thickened.

Add the crispy fried chicken pieces to the saucepan with the thickened orange sauce. Toss gently to coat each piece well, allowing the chicken to simmer in the sauce for about 1 minute.

Transfer the coated Orange Chicken to a serving dish. Garnish with sesame seeds, chopped green onions, and red pepper flakes if desired. Serve with steamed rice or stir-fried vegetables.