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Adjust rack to middle position and preheat oven to 425 degrees. Wash and dry all produce. Peel and halve carrot lengthwise; thinly slice crosswise into half-moons. Dice celery. Halve, peel, and dice onion. Mince or grate garlic. Trim, then thinly slice scallions, separating whites from greens. Halve ciabatta.