Place the orange, lemon, and ginger in a medium pot with 5 cups of water. Bring to a boil. Reduce the heat and simmer for 30 minutes.

Strain the water through a sieve into a bowl. Press the fruit into the sieve to extract as much water as released.

Return the water to the pot. Add the sugar, vinegar, ketchup, and a few drops of red food coloring if desired. Whisk until well blended. Place the pot over medium-high heat.

In a small bowl, combine the cornstarch and 3 tablespoons of cold water. Whisk until smooth.

Slowly stream the cornstarch slurry into the sauce, stirring constantly. When the sauce thickens, remove it from the heat.