Peel, cube and boil potatoes in salted water. Drain and mash with sour cream, milk, and butter. Add horseradish and mix well. Chop bacon and saute in olive oil. Cook until crisp. Remove bacon from pan and add finely chopped onion and finely diced garlic. Saute until lightly browned. Finely chop parsley. Add onion, garlic, bacon, parsley, cheese, salt, and pepper to mashed potatoes. Add more milk if mixture seems too thick. Transfer mixture to a buttered casserole dish. Lightly dot with additional butter and bake at 350 degrees until top is lightly browned. Cooking time does not include boiling potatoes.