Soak the clams in plenty of cold water for 1 hour, then drain, wash the clams and scrub the shells if they appear dirty. Arrange 4 large pieces of foil on a work surface and place a quarter of the clams in the centre of each one. Top each one with a quarter of the garlic, ginger, chilli, lime leaves and lemon grass and draw the edges of the foil up to form 'cups'.
Combine the coconut cream, lime juice and sugar and pour a quarter into each 'cup'. Seal the edges of the foil and cook the parcels on a hot barbecue for 8 minutes. Transfer the parcels to 4 plates, garnish with coriander and serve with crusty bread.