In a large Dutch oven, combine water, 1 tablespoon salt, and potatoes over medium-high heat. Bring to a boil; cook for 10 to 15 minutes or until tender. Drain potatoes well, and return to pan. Add sour cream and butter. Using a potato masher, mash potato mixture until butter melts. Add Parmesan cheese, horseradish, chives, parsley, remaining ½ teaspoon salt, and pepper, stirring to combine well. Garnish with parsley and chives, if desired.