Put the chillies, garlic, salt and sugar in a blender and whizz to a rough purée
2
Pour into a sterilised jar or bowl and cover. Leave to ferment for 3 days, removing the lid to allow the air to escape each day
3
Transfer to a blender and blitz again until very smooth. Add to a saucepan with the vinegar and fish sauce, bring to a simmer and cook for a few minutes
4
Taste and adjust seasoning, then pour into sterilised jars. The sauce will keep in the fridge for 2–3 months