In a small saucepan, add 1 cup sugar, 1 cup rice vinegar, and 1 cup water. Stir over medium heat until the sugar dissolves and the liquid looks clear.

Stir in 3-4 cloves of minced garlic, 1-2 tablespoons of chopped red chilies or 1-2 teaspoons of red pepper flakes, and 1 teaspoon of salt. If using fish sauce, add it now.

Bring the mixture to a gentle simmer and cook for 3-5 minutes until you smell the garlic mellow and see tiny bubbles on the surface.

In a small cup, whisk 1 tablespoon of cornstarch with 1 tablespoon of water to create a slurry. Slowly drizzle it into the simmering pot while stirring, continuing to stir for 1-2 minutes until the sauce turns glossy and lightly syrupy.

Turn off the heat and taste, adjusting with more vinegar for tang, sugar for sweetness, or extra chili for heat. The sauce will thicken a bit more as it cools.