Step 1.Place chillies and garlic in a food processor and pulse until roughly chopped.
Step 2.In a large saucepan over medium-high heat, combine the water, sugar, vinegar, fish sauce and salt. Add the chilli and garlic mixture and simmer until sugar dissolves. Then turn heat down and simmer for 5 minutes to let the flavours develop.
Step 3.Whisk in the potato starch mixture and gently simmer for another minute.
Step 4.Allow to cool, then store in a glass jar in the refrigerator for up to 3 weeks.
