Add the rutabaga into a pot of salted water. Bring it to a boil and cook it until tender.
Drain the rutabaga and using a hand mixer or potato masher, mash until perfectly smooth. Add the melted butter. Slowly add the warmed heavy cream until thick and creamy.
Transfer into a bowl and serve immediately.
Notes
TO STORE: Store leftovers in an airtight container in the refrigerator for up to 3 days. TO FREEZE: Let the mash cool completely, transfer it to a shallow container, and store it in the freezer for up to 2 months. Thaw overnight in the fridge before reheating. TO REHEAT: Microwave single-servings in 30-second intervals or heat in a small saucepan until warm.