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Adjust rack to middle position and preheat oven to 425 degrees. (Skip if microwaving in Step 5!) Wash and dry produce. Peel and slice onions into 1-inch-thick rounds; separate rounds into rings (discard small center pieces). Roughly chop parsley. Trim, peel, and cut carrots into sticks (like fries; ours were 3 inches long and ⅓ inch thick). Halve celery lengthwise.