Peel squash and potatoes and boil together in salted water until both are tender. Drain, reserving liquid. Sautee onions in butter until caramelized. Pour over potatoes and squash to mash, adding salt and pepper to taste and enough reserved liquid to achieve a smooth but not runny consistency.
Can be made ahead of time and frozen up to one month or refrigerated up to three days. Thaw overnight in fridge before baking in the over to reheat.
Tastes great with gravy or sour cream!