Buttermilk-Horseradish Mashed Potatoes

Everybody thinks of making mashed potatoes with milk, cream, half-and-half, or sour cream, but only in the South will you find them whipped with tangy buttermilk, just as they were when folks still churned their own butter. Horseradish is a much more modern innovation—and, in my opinion, a great one. I’d just as soon not eat mashed potatoes if they’re not served immediately after being beaten. Reheating them in a double boiler or wrapped in foil in the oven may keep them hot, but they’re st