This delicious casserole has become a tradition at my family's Thanksgiving. And everyone agrees we should make it more often, it's so good, and not hard to make. You can substitute mashed pumpkin or yams (unsweetened, of course!), but the butternut is best and worth the effort. A tip for easier clean up: I just mash the squash right in the casserole and mix everything there. I wipe off the upper sides with a clean, damp dish towel and pop it in the oven. I've been tempted to add a cup or two of cooked rice or other grain to make it even more hardy, but I haven't tried that yet...