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Adjust rack to top position (top and middle positions for 4 servings) and preheat oven to 425 degrees. Wash and dry all produce. Trim and thinly slice mushrooms. Trim and thinly slice scallions, separating whites from greens. Cut broccoli florets into bite-size pieces, if necessary. Dice potatoes into ½-inch pieces; place in a large pot with enough salted water to cover by 2 inches. Bring to a boil and cook until tender, 15-20 minutes. Reserve ½ cup potato cooking liquid, then drain and return potatoes to pot.