Preparation

• Peel and quarter the potatoes and cover with cold water. You can hold the potatoes this way at room temperature for 6 hours ahead of cooking (or for a day ahead in the refrigerator).
• Drain the potatoes, put them in a 6-qt. pot, and cover with water. Add 1 Tbs. salt. Bring to boil, reduce to a gentle boil, and cook until fork-tender, 25 to 30 minutes. Pour off the water and let the potatoes cool. (If they sit in water off the heat, they’ll turn pasty.)
• In an electric mixer with the whisk attachment, whip the potatoes until smooth, about 1 min.; add the butter and mix until melted and combined, about 30 seconds. Add the cream, 2 Tbs. salt, and pepper; whip until smooth and creamy, about 2 min., scraping down the sides of the bowl frequently. (You can also use a food mill.) Fold in the horseradish and combine well.
• To keep the potatoes warm— Put an inch of water in the pot the potatoes were cooked in and set over low heat. Put the potatoes in a mixing bowl, cover with foil, and put the bowl on top of the pot. You can hold the potatoes like this for at least 2 hours; just maintain the water level and keep the heat low.