Firstly toast the cummin & coriander seeds in a small pan for 3 or 4 minutes on a very low heat until the aroma is released. set aside.
Remove the course outer layer of the lemon grass. Remove as much of the tip and base that you need leaving only the white section. Slice into fine discs.
Using a mortar and pestle, grind the ingredients in the order they appear in the ingredient list until a paste develops . It will take 30 to 40 minutes depending on your stamina.
Divide the paste into 4 to 6 heaped tbs portions ( quantity will vary depending on the size of your chilies). Refrigerate for a few days or Freeze for up to 3 months.