Place the squash, potatoes, and garlic in a large saucepan and fill with enough water to cover the vegetables by 2″. Cover and place over medium-high heat. Bring to a boil, uncover, and add a generous pinch of salt. Reduce the heat to medium and cook until the squash and potatoes are tender, about 15-20 minutes.

Drain well and place in a large bowl. Add the butter, milk, heavy cream, salt and pepper to taste. Mash until the potatoes reach the desired consistency, adding more milk or cream if desired.

Stir in the fresh rosemary if using and adjust the seasoning if necessary.

Serve warm and enjoy!