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Adjust rack to top position (middle and top positions for 4 servings) and preheat oven to 425 degrees. Wash and dry produce.
Dice potatoes into ½-inch pieces. Zest and quarter lemon. Halve and core apple; grate one half on the largest holes of a box grater (use whole apple for 4). Trim and thinly slice scallions, separating whites from greens; mince whites.