Wash the crabs or lobsters and place on a hot charcoal grill or barbeque until red all over and cooked.
Remove the shell (and the stomach sac and gills in the case of the crab, these you discard) and chop into pieces. In either case, crack the claws. Arrange on a serving dish.
In a spice grinder, whiz the garlic, chillies, lime juice and fish sauce to se