In a medium sided bowl add the flour, 1/2 cup cornstarch, salt, and pepper whisk to combine now add in the egg, water, and 1 tbs of the oil whisk to combine. Now add the chicken in and stir to coat. Cover the bowl with cling wrap and refrigerate for 30 minutes.
In a deeper pan or large pot add the 3 ½ cups of vegetable oil and heat to 350 F (the oil needs to be good and hot before you add the chicken in) drip off and access batter off the chicken then add it into the oil. You will need to do this in batches so you don't overcrowd the pan. Cook the chicken for about 2-3 minutes on each side or until it is completely cooked. Once cooked remove the chicken to a separate plate lined with paper towels and set it aside.
In a small bowl add the 'cornstarch slurry ingredients and whisk to combine. Set to the side.
We are making the sauce now. In a deeper pan or pot on the stove add the orange juice, soy sauce, vinegar, brown sugar, and garlic bring this mixture up to a simmer while whisking, once simmering add the cornstarch slurry in and whisk until the sauce starts to get thick. Now add the cooked chicken in and coat the chicken in the sauce try to coat the chicken quickly so the chicken can be as crisp as possible.