Using a vegetable peeler, shave the zest from the orange in long, wide strips. If necessary, remove any large patches of bitter white pith from the zest strips with a paring knife. Juice the orange into a small bowl and mix with the soy sauce, rice vinegar, brown sugar, and red pepper flakes. Sprinkle the chicken with 1/2 tsp. of the salt. In a mini chopper or food processor, process the egg whites, cornstarch, and the remaining 1/4 tsp. salt until smooth. In a medium bowl, toss the chicken with the cornstarch batter. Heat 2 Tbs. of the oil in a 12-inch nonstick skillet or large stir-fry pan over medium-high heat until shimmering hot. Using tongs, transfer about half the chicken to the pan. Reduce the heat to medium and cook, flipping every minute or so, until the chicken browns and crisps all over and is firm to the touch, 3 to 4 minutes. With clean tongs, transfer to a paper-towel-lined plate. Add the remaining 1 Tbs. oil to the skillet (or 2 Tbs. oil if the pan seems very dry) and repeat the cooking process with the remaining chicken; transfer to the plate. Put the orange zest strips in the skillet and cook, stirring, until they darken in spots, 15 to 30 seconds. Stir the orange juice mixture and add it to the pan. Let it boil for about 10 seconds and then add the chicken and the scallion whites. Cook, stirring often, until the sauce reduces to a glaze and the chicken is just cooked through (check by cutting into a thicker piece), 1 to 2 minutes. If the chicken isn't cooked through but the glaze is cooking away, add a couple tablespoons of water and continue cooking. Serve sprinkled with the scallion greens.