In a large bowl, combine the chicken pieces with salt and pepper. In a separate bowl, whisk the eggs. In a third bowl, combine cornstarch and flour. Dip each piece of chicken first into the egg and then dredge in the cornstarch-flour mixture until well coated.

In a large frying pan or wok, heat vegetable oil over medium-high heat. Once hot, cook the chicken pieces in batches without overcrowding. Fry each piece for about 4-5 minutes, turning so all sides are golden and crispy. Remove with a slotted spoon and drain on paper towels. Discard used oil, leaving about 1 tablespoon in the pan.

In a saucepan, combine orange juice, sugar, soy sauce, rice vinegar, orange zest, garlic, ginger, and red pepper flakes. Bring to a simmer over medium heat, stirring to dissolve the sugar.

Stir the cornstarch slurry to recombine, then whisk it into the simmering sauce. Cook for 2-3 minutes, until the sauce thickens and becomes glossy.

Add the fried chicken to the pan with the reserved 1 tablespoon oil, then pour in the thickened orange sauce. Toss the chicken to coat evenly and cook for 1-2 more minutes, just until everything is hot.

Transfer the orange chicken to a serving platter. Sprinkle with sliced green onions and sesame seeds. Serve immediately, ideally with steamed rice.