We often use fresh breadcrumbs tossed with melted butter for this recipe, but we've also used potato chip crumbs, Ritz cracker crumbs, French fried onions, etc. Even panko would work. As a rule, breadcrumbs, panko and cracker crumbs really need to be tossed with a little melted butter, potato chip crumbs and French fried onions do not. They have enough fat already. The fat keeps the topping from drying out.