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Adjust rack to top position and preheat oven to 425 degrees. Wash and dry produce.
Dice potatoes into ½-inch pieces. Cut broccoli into bite-size pieces if necessary. Thinly slice scallions, separating whites from greens.
Place potatoes in a medium pot with enough salted water to cover by 2 inches. Bring to a boil and cook until tender, 10-12 minutes.
Drain, then rinse potatoes under cold water; transfer to a large bowl and refrigerate until ready to use.