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Adjust rack to top position and preheat oven to 425 degrees. Wash and dry produce.
Cut potatoes into ½-inch-thick wedges. Trim and thinly slice scallions, separating whites from greens; roughly chop whites. Quarter lemon. Trim and halve mushrooms (quarter any large mushrooms; skip if your mushrooms are pre-sliced!).
Cut broccoli florets into bite-size pieces if necessary. (Save potatoes for another use.)