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Adjust rack to top position and preheat oven to 450 degrees. Wash and dry produce.
Dice potatoes into ½-inch pieces. Halve and peel onion; grate one half on the largest holes of a box grater. Thinly slice remaining onion.
Place potatoes 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 servings), then drain.
Return potatoes to pot and mash with half the sour cream (you’ll use the rest later) and 2 TBSP butter (4 TBSP for 4) until smooth and creamy, adding splashes of reserved potato cooking liquid as needed. Season with salt and pepper.
Keep covered off heat until ready to serve.