These smooth, creamy, innocent-looking mashed potatoes deliver an exhilarating hit of horseradish. Prep and Cook Time: 1 1/4 hours. Notes: You can also make this with horseradish root, which can be wonderful if you find a good fresh one. Use 1/2 cup finely grated fresh horseradish, add it to the heated cream-milk mixture, and let steep off the heat for 30 minutes before adding it to the potatoes. The potatoes can be made a couple of hours ahead and then gently reheated in the microwave.