Mix the butter and garlic with a big pinch of salt. Heat the barbecue until the coals are ashy white. Lay a sheet of tin foil about 60cm long on the kitchen counter, put another sheet of the same size on top, then add a third sheet about 30cm long across the middle of the other sheets to make a cross shape. Spread the shallots in the middle of the foil, pile the mussels on top, dot the garlic butter all over, then scatter over half the parsley. Season, then fold the foil in at the sides to create an oval bowl shape.
Pour the wine into the foil bowl and then seal it by scrunching the foil together at the top. Make sure that it’s well sealed so that the mussels can steam – use an extra sheet of foil to wrap the whole parcel if necessary. Carefully place the parcel on the barbecue coals and cook for 10 mins. Open the parcel and check the mussels have opened up. Hot steam will billow out, so be careful. Pour in the cream, cover (if your barbecue has a lid) and allow to cook for a few mins longer, so the smoky scents of the barbecue can get in.
Sprinkle with the remaining parsley and serve with warm crusty bread.