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Wash and dry all produce. Adjust rack to middle position and preheat oven to 450 degrees. Cut potatoes into ½-inch-thick wedges. Halve and peel onion. Thinly slice one half; grate remaining half until you have 2 TBSP (save the rest for another use). Zest ½ tsp zest from limes, then cut each into quarters. Thinly slice tomatoes.