Prep Chicken: Cut the boneless skinless chicken thighs into 1-inch cubes and place them in a mixing bowl. In a separate bowl, whisk together the all-purpose flour, cornstarch, salt, and black pepper. Gradually mix in the egg, water, and a splash of vegetable oil until you achieve a smooth batter. Toss the chicken pieces in the batter, ensuring they are evenly coated, and let them marinate for 30 minutes to absorb flavor.

Fry Chicken: In a deep fryer or a heavy-bottomed pot, heat vegetable oil to 350°F (177°C). Carefully add the marinated chicken pieces in small batches, ensuring not to overcrowd the pan. Fry them until they turn golden brown and crispy, about 2-3 minutes per batch. Once cooked, remove the chicken and drain on paper towels.

Make Sauce: In a medium saucepan, combine the freshly squeezed orange juice, zest, soy sauce, rice vinegar, brown sugar, and minced garlic over medium heat. Stir the mixture well and bring it to a low boil, allowing the flavors to meld together.

Thicken Sauce: Mix cornstarch with a little water to create a slurry. Once the sauce is gently bubbling, whisk the cornstarch slurry into the saucepan. Continue to stir until the sauce thickens into a glossy consistency, about 1-2 minutes.

Combine: Add the crispy fried chicken pieces to the saucepan with the thickened Orange Chicken sauce. Gently toss the chicken until every piece is well-coated.

Serve: Transfer the orange chicken to a serving platter and garnish with optional sesame seeds, chopped green onions, and red pepper flakes. Serve immediately.