Place sliced parsnips and carrots in a large pot and add about 3 quarts of water and ½ tsp of salt. Bring to a boil over high heat. When boiling reduce heat to medium and cook until the veggies are tender, about 20 to 25 minutes.
Drain the vegetables and place in a large bowl with the butter. Mash with a potato masher until smooth. Add salt and pepper to taste. Sprinkle the parsley on top.
