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Adjust rack to top position (top and middle position for 4 servings) and preheat oven to 425 degrees. Wash and dry produce.
Dice potatoes into ½-inch pieces; place in a medium pot with enough salted water to cover by 2 inches. Bring to a boil and cook until tender, 15-20 minutes. Reserve ½ cup potato cooking liquid (¾ cup for 4), then drain.
Return potatoes to pot and mash with garlic herb butter until smooth and creamy, adding splashes of reserved potato cooking liquid as needed. Stir in half the sour cream and season with salt and pepper. Keep covered off heat until ready to serve.