In a large saucepan combine potatoes with salted water to cover by 2 inches and simmer until very tender, about 50 minutes. Drain potatoes. Return potatoes to pan and heat over low heat, shaking pan, until dry. Cool potatoes until they can be handled.

Peel potatoes and force through a ricer or medium disk of a food mill into pan. Add butter, garlic, and milk and cook mixture over moderately low heat, beating constantly with a wooden spoon, until fluffy and heated through, about 2 minutes.

Add mozzarella and salt and pepper to taste and cook mixture, beating until cheese is melted and forms long elastic strands when lifted with spoon.

Divide aligot among 6 buttered 1-cup shallow gratin dishes or one 6-cup buttered gratin dish. Aligot may be prepared up to this point 1 day ahead and chilled, covered. Bring aligot to room temperature before proceeding with recipe.

Preheat broiler.

In a bowl with an electric mixer beat cream until it holds soft peaks and beat in horseradish and salt to taste. Spread horseradish cream over aligot and broil about 4 inches from heat until golden, about 1 minute.

Per serving: 435 Calories (kcal); 41g Total Fat; (82% calories from fat); 7g Protein; 13g Carbohydrate; 118mg Cholesterol; 96mg Sodium Food Exchanges: 0 Grain(Starch); 0 Lean Meat; 1 Vegetable; 0 Fruit; 8 Fat; 0 Other Carbohydrates