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Adjust rack to middle position and preheat oven to 400 degrees. Wash and dry all produce.
Halve and peel onion; cut half into ½-inch thick wedges and very thinly slice remaining onion. Cut tomato into ½-inch-thick wedges. Core, deseed, and dice bell pepper into ½-inch pieces. Zest and halve lemon. Peel and grate garlic.