Skim 1/4 cup of the thick cream from the top of the coconut milk and set remaining milk aside.
Warm oil and 1/4 cup coconut cream in a 10-inch cast iron skillet over medium heat until simmering. Add curry paste, increase heat to medium-high and stir-fry until oil separates and paste is slightly browned. Lower heat to mudium and stir in fish sauce, brown sugar and salt. Add reserved coconut milk, lemongrass and lime juice. Bring to a boil, stirring occasionally.
Add mussels and prawns and cook 3 to 4 minutes. Then add halibut and continue cooking until the mussels open, the prawns turn pink and halibut is white throughout, about 5 minutes.
Remove lemongrass. Scatter the cilantro and basil over the top. Serve in individula bowls over cooked Jasmine rice, with lime wedges on the side.