Place a third of the potatoes in a shallow 3-qt. baking dish coated with cooking spray. In a small bowl, combine the flour, salt and pepper; sprinkle half over potatoes. Sprinkle with 1/4 cup of each cheese and half of the onions. Repeat layers. Top with remaining potatoes. Pour milk over all., Cover and bake at 350° for 50-60 minutes or until potatoes are nearly tender. Sprinkle with remaining cheeses. Bake, uncovered, 10 minutes longer or until cheese is melted and potatoes are tender. Sprinkle with parsley.