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Preheat oven to 500 degrees or broiler to high with rack 3-4 inches from flame. Bring a large pot of salted water to a boil. Wash and dry all produce. Trim zucchini, then slice into ⅓-inch-thick rounds. Halve, peel, and finely dice onion. Trim and thinly slice scallions, separating whites from greens. Mince or grate garlic.