This holiday season, skip the usual marshmallow-topped casserole and serve this Savory Sweet Potato Casserole for a change of pace. Because sweet potatoes are so naturally sweet, they are nicely balanced by this savory Parmesan-herb streusel topping. The topping is made with day-old sourdough bread, but you can use another type of stale white bread such as country white, if you prefer. To cut down on sticky clean up, place the potatoes on a foil-lined baking sheet before roasting them. We promise, no one will miss the marshmallows.