Aligot Gratin with Horseradish Cream

Cover potatoes with salted water by 2 inches in a 4-5 quart heavy pot and simmer until very tender, about 40 minutes. Drain in a colander. Mince garlic and mash to a paste with 1 teaspoon salt using the side of large heavy knife. When the potatoes are cool enough to handle when held in a kitchen towel, peel and force through ricer into clean saucepan. Add butter, garlic paste, and milk and cook over medium-low heat, stirring vigorously with a wooden spoon, until fluffy and heated through, about 2 minutes. Add cheese and pepper and cook, stirring vigorously with a heatproof rubber spatula or wooden spoon, until cheese is almost melted, smooth, and almost taffy-like, about 10 minutes. Preheat broiler. Butter baking dish(es). If using 6 dishes, put onto baking sheet. Transfer potato mixture to baking dish, smoothing top. Beat cream in a bowl with an electric mixer until it hold soft peaks, then beat in horseradish and remaining ¼ teaspoon salt. Spread horseradish cream over potato mixture and broil 4-5 inches from heat, rotating gratin to brown evenly, until golden, 1-2 minutes.