Combine 2 1/2 pounds whole unpeeled beets and 1 large unpeeled russet potato in a pot; cover with cold water. Bring to a boil and cook until soft, about 45 minutes; drain. Peel the beets and potato and puree with 3 tablespoons butter; season with salt. Top with horseradish sauce.
Use a food processor or immersion blender to puree these veggies-they're too fibrous for a potato masher.