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Adjust racks to top and middle positions and preheat oven to 450 degrees. Bring 1 TBSP butter (2 TBSP for 4 servings) to room temperature. Wash and dry produce.
Cut potatoes into ½-inch-thick wedges. Halve, peel, and finely dice half the onion (whole onion for 4). Halve baguettes lengthwise, stopping before you get all the way through.