Measure 1 cup orange juice and set aside 2 tablespoons of it in a small bowl for the cornstarch slurry. Put the remaining orange juice into a medium saucepan. Add 1/4 cup white sugar, 1/4 cup brown sugar, 2 tablespoons rice vinegar, 1/4 cup soy sauce, 1 tablespoon minced fresh ginger, 2 cloves garlic (finely diced), and 1/2 teaspoon red pepper flakes. Stir to combine.

Cook the mixture over medium heat, stirring occasionally, until it comes to a gentle simmer; continue cooking for 3 minutes to dissolve the sugars and blend the flavors.

While the sauce simmers, whisk 1 tablespoon cornstarch into the reserved 2 tablespoons of orange juice until fully dissolved and smooth.

Slowly pour the cornstarch slurry into the simmering sauce while whisking continuously. Continue to cook over medium heat, whisking frequently, until the sauce thickens, about 2–5 minutes.

Remove the pan from the heat and stir in the zest of 1 orange. Let the sauce cool slightly before using.