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Wash and dry all produce. Preheat broiler to high or oven to 500 degrees. Halve zucchini lengthwise, then cut into long, thin wedges. Halve and peel onion, then cut into 1-inch-thick wedges. Finely mince 1 wedge. Zest lemon until you have 1 tsp zest, then cut into wedges. Halve tomatoes. Pick basil leaves from stems; discard stems. Thinly slice leaves.