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Adjust rack to middle position (top and middle positions for 4 servings) and preheat oven to 450 degrees. Wash and dry produce.
Halve potatoes lengthwise. Strip rosemary leaves from stems; finely chop leaves until you have ¼ tsp (½ tsp for 4). (We sent more; save the rest for another use!) Halve, peel, and thinly slice onion. Zest and quarter lemon. Trim and discard woody bottom ends from asparagus.