Blend all the ingredients for the paste until smooth, and transfer to a saucepan. Bring to the boil and simmer, stirring often, for 5 minutes. Add the coconut milk, lemon grass and kaffir lime leaves. Bring everything to the boil again, then simmer for 50 minutes, stirring occasionally.
Add the chopped tomatoes and some more salt. Go on simmering for another 10 minutes. Adjust the seaso