Bring potatoes and carrots to boil in 2 quarts of salt-seasoned water over medium-high heat in a large, covered pot. Continue to cook until tender, about 10 minutes. Holding lid ajar over the pot, drain water. Using a potato masher, coarsely crush potatoes and carrots, stirring in scallions and creamer, followed by butter. Can be made a day or two ahead. Reheat in the microwave or in a heat-proof bowl set over a pan of simmer water.