Remove the zest from the orange using a zester or small-holed cheese grater, then squeeze the juice from the orange. You’ll need about 1 tsp zest and 1/2 cup juice. In a small bowl, combine the juice and 1 tsp zest with the soy sauce, brown sugar, rice vinegar, grated ginger, minced garlic, red pepper flakes, and 1/2 Tbsp cornstarch. Whisk the ingredients together until the cornstarch is fully dissolved. Set the sauce to the side.
Use a sharp knife to remove any excess fat from the chicken thighs, then cut them into small 3/4-inch pieces.
Combine the egg, 2 Tbsp cornstarch, and a pinch of salt and pepper in a mixing bowl and whisk until the mixture is smooth and frothy. Add the chicken pieces and stir to coat them in the egg mixture.
Heat a large skillet over medium to medium-high heat. Once hot, add 2 Tbsp cooking oil and swirl to coat the bottom of the skillet. Add the chicken, making sure all the pieces are touching the surface and not piled on top of one another. Let the pieces cook until they are golden brown on the bottom.
Flip the chicken pieces, breaking them apart from one another as you turn them. Let them cook until golden brown on the second side and cooked through (about 5-7 minutes total cooking). Transfer the cooked chicken to a separate plate.
Pour the prepared orange sauce into the same skillet and turn the heat down to medium-low. Give the sauce a stir and allow it to come up to a simmer, at which point it will start to thicken and turn glossy. Continue to simmer for 2-3 minutes.
Add the cooked chicken pieces back into the skillet and stir to coat the chicken with the sauce. Heat for 1 more minute.
Serve the chicken and sauce with cooked rice and garnish with sliced green onions and any leftover orange zest.
*Rice vinegar gives the sauce the perfect tangy flavor without a strong vinegar aftertaste or aroma. You can find rice vinegar in the Asian section of most major grocery stores.**I use chicken thighs because they're budget-friendly and always stay juicy and tender, but you can use chicken breast if you prefer. Just be careful not to over cook the chicken breast as it can become tough and dry.***Use a neutral cooking oil that can handle higher temperatures, like canola, peanut, vegetable, or light olive oil.