Put the turnips and potatoes in a medium saucepot and cover with cold water. Add a generous pinch of salt and bring to a boil. Reduce the heat to maintain a simmer and cook until the vegetables are tender and meet no resistance when pierced with a fork, about 20 minutes.
Drain well. Whip in a large bowl using an electric mixer on medium-high speed until well blended. Add the sour cream, mustard and butter and continue to whip on medium speed until smooth and creamy. Adjust the seasoning with salt and pepper. Sprinkle with the parsley and top with a pat of butter.
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