As widely as this is thought to be true, au gratin does not mean covered with a thick layer of Day-Glo melted cheese. All it means is that there’s some kind of crust on top. In this recipe, the crust is mostly bread crumbs, although there’s some Parmesan cheese both in the crust and in the sauce. You will thank me for not ruining the taste of the crabmeat with melted Cheddar or the like.