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Adjust rack to top position and preheat oven to 450 degrees. Wash and dry all produce. Butter a 2-quart baking dish (we used an 11-by-8-inch dish; use a 3-quart or 13-by-9-inch dish for 6 servings); set aside until step 4. Arrange bacon in a single layer on a baking sheet. Bake on top rack until crispy, 15-20 minutes. Meanwhile, trim and thinly slice mushrooms. Halve, peel, and dice onion. Pick basil leaves from stems; roughly chop half the leaves and reserve remaining. Tear ciabatta into ¾-inch pieces.