Split the roll and place cheese on bottom half. Lightly broil the half with the cheese in the toaster oven if desired. Meanwhile, spray a microwave-safe ramekin or coffee mug generously with cooking spray. Pour egg whites into the dish and cover. Microwave for 45-60 seconds, depending on your microwave. Place the tomato slice on top of the cooked egg and nuke it for 10 more seconds (to get the chill of the refrigerator off).