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Adjust rack to middle position and preheat oven to 425 degrees. Wash and dry produce.
Halve, peel, and dice onion into ½-inch pieces. Trim and halve zucchini lengthwise. Lay flat and slice into ¼-inch-thick planks; reserve 6-8 of the widest zucchini planks (you’ll use them to top the “lasagna” before baking). In a shallow dish, toss remaining zucchini planks with half the flour until coated.