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Adjust rack to top position (top and middle positions for 4 servings) and preheat oven to 450 degrees. Bring 5 cups water (9 cups for 4) to a simmer in a medium pot. Wash and dry produce.
Thinly slice mushrooms (skip if your mushrooms are pre-sliced!). Trim and quarter zucchini lengthwise; cut crosswise into ½-inch- thick pieces. Thinly slice scallions, separating whites from greens; mince whites. Peel and mince or grate garlic. Roughly chop parsley.