Soak the chillies in warm water for about 30 minutes and allow to drain. If you want a milder paste then remove the seeds alternatively, leave them in for a spicier taste.
Roughly chop the onions, garlic, ginger and lemon grass and add them with all the rest of the ingredients to a food processor.
Process until a smooth paste is achieved. If you have left the seeds in it will take a bit longer. I had to process it for about 20 minutes until smooth.
The paste is now ready to use but it will keep in the fridge for a few days if necessary, alternatively it can be frozen.