Cook potatoes in boiling water to cover 20 minutes or until almost tender. Drain and cool slightly. Peel potatoes, and cut into 1/4-inch-thick slices. Arrange potato slices in a lightly greased 13- x 9-inch baking dish. Sprinkle with 1/2 teaspoon salt and 1/2 teaspoon pepper.
Stir together half-and-half and horseradish, and pour over potato.
Bake, covered, at 400° for 40 minutes.
Melt butter in a large skillet over medium heat. Add onion, remaining1/2 teaspoon salt, and remaining 1/2 teaspoon pepper; cook, stirring occasionally, 20 minutes; add sugar, and cook, stirring occasionally, 5 to 8 minutes or until onion is caramel colored.
Stir in vinegar, and cook 2 minutes or until liquid evaporates. Remove from heat, and cool 5 minutes. Fold in cheese and 2 tablespoons parsley.
Uncover potato; top with onion mixture. Reduce temperature to 350°.
Bake at 350° for 30 minutes. Let stand 5 minutes; sprinkle with remaining 2 tablespoons parsley.