Cut the chicken into bite-sized 1-inch pieces. In a mixing bowl, combine the flour, cornstarch, salt, and pepper. Whisk in the egg, vegetable oil, and water until you get a smooth batter.
Submerge the chicken chunks into the batter to coat each piece completely. Cover the bowl and refrigerate for about half an hour, or at least 10 minutes if you’re in a hurry, to help the batter firm up.
Heat about two inches of vegetable oil in a deep pan or fryer to 350°F (177°C). Line a plate with paper towels to drain the fried chicken later.
Fry the battered chicken in small batches to avoid crowding the pan. Cook each batch for 2 to 3 minutes until golden and crispy, turning occasionally for even cooking. Remove the chicken and let it drain on the paper towels. Keep warm by tenting with foil.
In a separate saucepan over medium heat, mix orange juice, orange zest, soy sauce, vinegar, brown sugar, and minced garlic. Bring this to a gentle boil, tasting and adjusting the flavor to your liking.
Slowly stir the cornstarch slurry back together and pour it into the simmering sauce, stirring constantly. Continue until the sauce thickens to coat the back of a spoon, which should take from half a minute to a few minutes.
Add the crispy chicken to the thickened orange sauce and toss until every piece is nicely coated with the glaze.
Serve hot with a sprinkle of sesame seeds, chopped green onions, and a pinch of red pepper flakes if desired, to enhance both flavor and presentation.
