Wash and dry potatoes, then cut into evenly-sized pieces. Place into a large pot of cold, salted water and bring to a boil. Reduce heat and cook potatoes until fork tender. Remove from heat, drain and transfer to a large bowl.
Add butter, sour cream, Parmesan cheese and horseradish. Lightly mash – do not over mash – and add parsley. Season with salt and pepper. Serve and enjoy!