Use a 4 to 6-quart slow cooker for best results. For a crispier top, broil the cooked casserole in the oven for 2-3 minutes before serving. Do not open the slow cooker frequently while cooking to maintain heat. This recipe is naturally gluten-free and can be customized for vegetarian or dairy-free diets. Leftovers can be refrigerated for 3-4 days or frozen for up to 2 months.