Preparation Combine Jerusalem artichokes and potatoes in large pot. Pour enough cold water over to cover; add 1 tablespoon coarse salt. Bring to boil; reduce heat and boil gently until all vegetables are tender when pierced with knife, about 18 minutes. Drain, reserving cooking liquid. Return vegetables to pot. Mash vegetables, adding reserved cooking liquid by 1/2 cupfuls to moisten until chunky mixture forms. Stir in butter. Season to taste with salt and pepper. Transfer to bowl and serve. DO AHEAD: Can be made 2 hours ahead. Transfer mash to large heatproof bowl. Let stand at room temperature. Rewarm in same bowl set over simmering water, stirring occasionally, before serving.
Preparation Combine Jerusalem artichokes and potatoes in large pot. Pour enough cold water over to cover; add 1 tablespoon coarse salt. Bring to boil; reduce heat and boil gently until all vegetables are tender when pierced with knife, about 18 minutes. Drain, reserving cooking liquid. Return vegetables to pot. Mash vegetables, adding reserved cooking liquid by 1/2 cupfuls to moisten until chunky mixture forms. Stir in butter. Season to taste with salt and pepper. Transfer to bowl and serve. DO AHEAD: Can be made 2 hours ahead. Transfer mash to large heatproof bowl. Let stand at room temperature. Rewarm in same bowl set over simmering water, stirring occasionally, before serving.