Most of the time, mashed parsnips and pureed parsnips are the same dish. Parsnips are much less starchy than potatoes, so they do not become as gloopy when pureed in a food processor or blender.  If you are looking to physically mash your parsnips, you can still boil them down and pass them through a ricer or metal sieve. Roasted parsnips will be very hard to mash as they do not break down easily without liquid.