Preheat oven to 450°. Toss potatoes with 1/4 cup Hellmann's® or Best Foods® Real Mayonnaise, then arrange in single layer on baking sheet. Roast 35 minutes or until golden brown and tender, turing potatoes once. Arrange potatoes on serving platter.
Meanwhile, combine milk, remaining 1/4 Hellmann's® or Best Foods® Real Mayonnaise, mustard, garlic, pepper and cheese in microwave-safe bowl with wire whisk. Microwave at HIGH 1 minute, stirring once halfway through. Stir (mixture will be lumpy), then drizzle cheese sauce over potatoes just before serving. Sprinkle with green onions.