Preheat oven to 450 degrees F. In 6-quart saucepan, place potatoes and enough water to cover, heat to boiling over high heat. Reduce heat to low, cover and simmer 15 minutes or until potatoes are fork-tender, drain.

Meanwhile, reserve 1/4 cup sour cream and 1 tablespoon chives to use later in topping.

Return potatoes to saucepan. With potato masher, mash potatoes with milk, salt, remaining sour cream, and chives. Spoon potato mixture into shallow 2 1/2-quart casserole.

In small bowl, stir mayonnaise with horseradish, pepper, reserved sour cream, and reserved chives. Spread horseradish mixture over mashed potatoes.

Bake potatoes 20 minutes or until hot and bubbly and top is lightly browned. Garnish with chives.