Fill a large pot or pasta pot with about 2 inches of water. Place a steamer basket into the pot, making sure it sits just above the water. Add the potatoes and garlic to the basket, cover the pot and turn the heat to medium. Steam the potatoes until fork tender, 15 to 20 minutes.
Meanwhile, combine the cream and butter in a small saucepan. Heat over medium-low heat until the cream is hot and the butter melted.
When the potatoes are done, remove the steamer basket and dump out the water from the pot. Add the potatoes to the empty pot and mash with a potato masher. Gently stir in the hot cream mixture and season to taste with salt and pepper. Just before serving, mix in the horseradish. Transfer to a serving bowl and serve.