Potatoes au ___ (6)
I believe the answer is:
gratin
(Other definitions for gratin that I've seen before include "A dish cooked with grated cheese on it" , "Grated cheese or breadcrumbs on a dish" , "'Au ..., with a cheesy topping (6)'" , "With a crisp, golden topping" , "Cooked covered in grated cheese or breadcrumbs" .)