Put lemongrass, chili, ginger, kaffir lime leaves, coriander roots, ground cumin, ground coriander, fish sauce, sugar, lime juice into a food blender, and blend together until smooth. Adjust the seasoning of the paste to your own taste.
Heat oil in a large sauce pan, make sure the sauce pan has a lid. When the oil is hot, add the curry paste into the pan, and fry it for a couple of minutes.
Add coconut milk, and bring it to boil. Once boiled, add the mussels, cover the pot with the lid, and turn the heat down to medium. Let it simmer for 3-5 minutes or until the mussels have all opened.
Turn the heat off, and divide the mussels into two portions. Sprinkle the fresh coriander on the top and serve immediate.