Peel and chop 1 pound parsnips and 1 large russet potato; put in a saucepan with 2 garlic cloves. cover with water, add kosher salt and bring to a boil; cook until soft, 20 minutes. reserve 1/2 cup cooking water, then drain the vegetables and return to the pan. add the reserved cooking liquid, 1/3 cup sour cream, 3 tablespoons butter, 1 teaspoon salt, and nutmeg to taste. beat with a mixer until smooth. season with pepper.