Set 1/4 cup of the bacon aside for garnish. Gently fold mayonnaise, 1 1/2 cups of the cheese and remaining bacon into the mashed potatoes. Spoon mixture into baking dish. Bake uncovered 25 minutes. Sprinkle top with reserved bacon and remaining 1/2 cup cheese. Bake 5 minutes longer or until cheese is melted. Sprinkle with green onions.