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Adjust racks to top and middle positions and preheat oven to 400 degrees. Wash and dry all produce. Butter a 2-quart baking dish (or 8-by-8-inch dish).
Dice ciabattas into ¾-inch pieces. Halve, peel, and dice shallot. Trim and discard woody bottom ends from asparagus; slice crosswise on a diagonal into ½-inch-thick pieces. TIP: If asparagus is thicker than a pencil, halve lengthwise first.