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Adjust racks to top and middle positions and preheat oven to 450 degrees. Wash and dry produce.
Cut potatoes into ½-inch-thick wedges. Halve, peel, and thinly slice onion. Halve, core, and thinly slice green pepper. Trim and slice mushrooms into ¼-inch-thick pieces (skip if your mushrooms are pre-sliced!).
Cut broccoli florets into bite-size pieces if necessary. (Save potatoes for another use.)