Add orange juice, white and brown sugar, rice vinegar, soy sauce, ginger, garlic, and red chili flakes to a medium saucepan. Stir and cook for 3 minutes over medium heat.
Whisk one tablespoon of cornstarch with 2 tablespoons of water to form a slurry in a small bowl. The cornstarch should be fully dissolved.
Add the slurry to the orange sauce and whisk together. Continue to cook for 5 minutes until the mixture begins to thicken.
Once the sauce is thickened, remove from heat and stir in orange zest.
