Combine crab meat, squid and prawn pastes, water chestnuts and bird's eye chillies in a mixing bowl.
Add onion, coriander, kaffir lime leaves and egg. Mix in seasoning ingredients. Stir to combine.
Sprinkle some cornflour on the inside of the crab shell. Shake off excess flour and spoon in 2-3 tbsp filling mixture. Stuff filling into the corners of the shells. Press down firmly, then sprinkle a little extra cornflour over the filling.
Deep-fry the prepared shells in hot oil until the filling is cooked through. Dish out and drain from oil.