Preheat the oven to 350°F and grease a 9×11-inch baking dish and set aside. Melt 3 tablespoons of butter in a large pot or deep skillet and add the chopped onion. Saute until they are softened and translucent.
Storage Tip: Place leftover Carrot Casserole into an airtight container and place it in the refrigerator. Eat the leftovers within 3 days! If you’d like you could also freeze them for up to 3 months.