Place the cubed potatoes, rutabaga and parsnips into a large pot and add enough water to cover the vegetables.

Bring to a medium boil and cook, uncovered, until tender and easily pierced with a fork, about 20 minutes. Drain the water from the pot.

In a small saucepan, warm the butter, sour cream and milk until the butter is fully melted and the sour cream is well incorporated.

Add milk mixture, garlic horseradish, salt and pepper to the potatoes.

Mash until smooth using a potato masher and then transfer to a serving dish.