Halve the langoustines lengthways and remove the stomach sac and the intestinal tract.*They are easiest to halve if you lay them on their backs and then make one firm, decisive cut along the length.
Melt the butter in a saucepan and let it foam up well.
Chop the parsley and grate or crush the garlic, then stir both into the butter and turn off the heat.
Lay the langoustines shell side down on a very hot barbecue. Brush or spoon the butter mixture over each langoustine and grill them over direct heat for about 3-4 minutes, depending on size.*Save any butter mixture that is not used.*You can also put the langoustines in a foil tray, pour the butter mixture over them and set the tray on the barbecue.
Grill the lemon halves too, cut side down.
Serve the grilled langoustines on a large platter and squeeze plenty of juice from the grilled lemons over them. Pour over the last of the butter as well, if there is any left.