Preheat oven to 400 degrees F (200 degrees C). On a tray, roast squash (skin side up)and potatoes until squash has browned and softened and potatoes are cooked through soft (you may see some sugars seeping out). During the last 15 minutes of roasting add the bananas to the tray (skin on) and let them roast until the skin turns almost black.
Put all in a large bowl and cover with foil and let sit for 15 minutes. (This will steam the skins off.) Peel the skins off the squash, potatoes and bananas and mash together in the bowl (you can leave it chunky or smooth as you like).
Add remaining ingredients except marshmallows, and combine completely. Pour into buttered casserole and top generously with marshmallows (the kids will love to do this part)
Bake 15 to 20 minutes until marshmallows are melted and bubbly.
The bananas are the secret ingredient that gives this dish an unusual sweet and rich flavor that nobody can figure out (until now).