Don't be fooled by the simple presentation and title of this incredible recipe there's some serious technique on show. The halibut is portioned and brined, with all the trimmings used to make a rich mousse studded with mussels that's spread on top before steaming. The sauce, made from the mussel cooking liquor, is thickened with egg and seaweed butter to create a silky custard texture, and the potatoes are cooked confit in olive oil. Quite simply stunning.