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Adjust rack to top position and preheat oven to 425 degrees. Wash and dry all produce. Cut potatoes into ½-inch-thick wedges. Trim and thinly slice scallions, separating whites from greens; finely chop whites until you have 1 TBSP (2 TBSP for 4 servings). In a small bowl, combine mayonnaise, mustard, chopped scallion whites, half the sour cream, half the paprika, and 1 tsp vinegar (2 tsp for 4). Season with salt and pepper. Set aside.