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.
TO STORE: Store leftovers in the refrigerator, covered, for up to 3 days. TO FREEZE: Place the cooled keto mashed potatoes in an airtight container and store it in the freezer for up to 2 months. TO REHEAT: Either microwave the mash in 30-second spurts until warm or in a small saucepan over stovetop until it is hot.