In two greased 13x9-in. baking dishes, layer potatoes and onions. Combine the flour, soup and milk until blended; pour over each baking dish. Season with salt and pepper. Cover and bake at 325° for 1-1/4 to 1-1/2 hours or until potatoes are tender. Uncover; cool for 30 minutes. Cover and refrigerate overnight. Remove from refrigerator 30 minutes before reheating. Bake at 325° for 30-40 minutes. Uncover and sprinkle with cheese; bake 5 minutes longer or until cheese is melted.