How to Make It
In a medium bowl, combine the heavy cream, garlic, salt and pepper. Gradually stir in the horseradish, tasting for strength.
In a 13-by-9-inch nonreactive baking dish, layer the potatoes and apple. Pour the horseradish cream over the potatoes. Bake in the upper third of the oven for about 30 minutes, or until bubbling. Sprinkle the Parmesan cheese over the top and bake for about 15 minutes longer, or until the potatoes are tender and the top is nicely browned. Cover the gratin with foil and let stand in a warm place for 10 to 30 minutes before serving. Cut the gratin into 8 squares with a pie server.