Make Ahead: Prepare the casserole 1-2 days in advance. First make the filling, and then smooth it into the baking dish. Do not add the topping. Cover and refrigerate. When you’re ready to bake the casserole, take it out of the fridge and allow the dish to come to room temperature for about 30-60 minutes. Cover and bake at 375°F for about 10-15 minutes (or until the potatoes are nice and warm). Remove the cover, add the pecans and marshmallows on top, and return the casserole to the oven for about 15 more minutes (or until the marshmallows are golden brown).