Blend the garlic, chilli, lemon grass, ginger, galangal, lime leaves and coriander to a rough paste in a mini food processor.
Mix the vinegar, fish sauce and soy sauce together in a small bowl.
Put the sugar in a saucepan with the water and stir over medium heat until the sugar has dissolved. Turn the heat to high and cook for 4 minutes, or until the sugar begins to colour. Keeping constant watch, continue boiling the suggar until it is almost burning and has a wonderful toffee smell.
Quickly add the spice paste and stir for 1 minute. Stir in the vinegar mixture and cook for a further 2 minutes, then take the pan off the heat. Allow to cool, pour into a clean glass jar, cover and refrigerate until ready to use.
The sauce will keep for several weeks in the refrigerator.