Microwave butter in 2-quart casserole 50-60 seconds or until melted. Stir in horseradish, lemon juice, salt and pepper. Stir in potatoes until well coated with butter. Cover; microwave, stirring after half the time, 10-12 minutes or until potatoes are fork tender. Let stand 5 minutes. Garnish with lemon slices and sprinkle with parsley.