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Adjust rack to top position and preheat oven to 450 degrees. Wash and dry produce.
Trim and halve zucchini lengthwise; cut crosswise into ½-inch-thick half-moons. Halve, peel, and cut onion into ½-inch-thick wedges; mince one wedge until you have 1 tsp (2 tsp for 4 servings). Cut tomato into ½-inch-thick wedges. Zest and quarter lemon.