In a strict, classical French-cooking sense, “À la Nantaise” means with mussels, mushrooms, truffles and Hollandaise sauce, such as in the dish called Coquilles (“Scallops”) Saint-Jacques à la Nantaise.
There are, though, many other working definitions of it that are completely as different to each other as they are to the classical sense. Sometimes, for instance, it means with wine sauce, small onions and prunes. “Lapin en gelée à la Nantaise” is a terrine made from rabbit baked in wine and small rounds of onions and carrots. The wine, though it is white in that particular recipe, is Muscadet, which hails from around Nantes.
The reference is to the city of Nante in the Loire Valley of France.