In a large bowl, combine the egg, 1/2 cup cornstarch, water, flour and salt. Mix until combined. Add the chicken and stir until the chicken is completely coated.

Preheat the oven to 200ºF. Place a cooling rack on a baking sheet and set aside.

Pour about 2 inches of vegetable oil into a heavy skillet. Heat over medium-high heat.

Once the oil is hot, add about 1/4 of the chicken to the hot oil in a single layer.  Cook for about 2-3 minutes, then turn the chicken to cook on the second side. Fry until the chicken is cooked through. Once cooked, transfer the chicken to the baking sheet and place in the oven to keep it warm while you cook the rest of the chicken.

Working in batches, cook the remaining chicken.

While the chicken is cooking, make the sauce.

In a saucepan, combine the orange juice, soy sauce, sugar, mirin, rice vinegar, ginger, sesame oil and red pepper flakes. Set over medium heat.

In a small bowl, whisk the 2 tablespoons of cornstarch and 2 tablespoons of water together.

When the sauce comes to a boil, whisk in the cornstarch mixture. Continue to cook until the sauce has thickened, then lower the heat to keep warm until the chicken is cooked.

When all of the chicken is cooked, place it in a large bowl. Pour the sauce over the top and stir to coat.

Serve topped with green onions, if desired.