Preparation Combine potatoes, parsnips, parsley root, if using, and garlic in heavy large saucepan. Add 3 cups water and broth; if necessary, add additional water to cover by 1 inch. Boil uncovered until vegetables are tender, about 25 minutes. Drain vegetables, reserving liquid. Return vegetables to pot. Add cream and butter. Set over low heat; mash until mixture is smooth; and fluffy, adding enough reserved cooking liquid to thin to desired consistency. Mix in 2 tablespoons parsley. (Can be made 2 hours ahead. Cover; let stand at room temperature. Rewarm over low heat, stirring.) Transfer to bowl. Top with additional parsley.
Preparation Combine potatoes, parsnips, parsley root, if using, and garlic in heavy large saucepan. Add 3 cups water and broth; if necessary, add additional water to cover by 1 inch. Boil uncovered until vegetables are tender, about 25 minutes. Drain vegetables, reserving liquid. Return vegetables to pot. Add cream and butter. Set over low heat; mash until mixture is smooth; and fluffy, adding enough reserved cooking liquid to thin to desired consistency. Mix in 2 tablespoons parsley. (Can be made 2 hours ahead. Cover; let stand at room temperature. Rewarm over low heat, stirring.) Transfer to bowl. Top with additional parsley.