Combine the fish sauce, rice vinegar, sugar, and red chili flakes together in a medium sized pot over medium-high heat. Whisk to dissolve the sugar, and bring the mixture to a boil (about 10 minutes). Keep an eye on the pot – if the heat is too high there is a chance it may boil over if you aren’t careful (not that I know from experience or anything…). In the meantime, whisk together the cornstarch and the water in a separate bowl.

Once the mixture has reduced by a third or so, reduce the heat to low and whisk in the cornstarch mixture. Allow the mixture to thicken, stirring occasionally (about 2 minutes). Remove the sauce from heat and allow it come to room temperature. Once cool, transfer the contents to a glass jar or bottle and store in the fridge. Use within 3-4 weeks.