First, cut the lemongrass in half lengthwise and bash it with the flat of a knife a few times to release the oils.
Heat the oil in a saucepan over low-medium. Add the curry paste, stirring until aromatic for about 30 seconds. Add the coconut milk, lemongrass, lime leaves, sugar, fish sauce and chopped tomatoes. Simmer a few minutes.
Increase heat and bring to a boil. Add all the mussels, cover the pan and cook 3-5 minutes until the mussels are open. Discard any that do not open.
Serve garnished with coriander and basil leaves and with white rice and lime wedges on the side.