In a large bowl, combine the soup, water, milk, mustard, and pepper until blended. In a nonstick skillet, celery and onions in oil until celery is tender. Stir the vegetables, tuna, egg substitute and noodles into the soup mixture. Pour into a 2-qt. baking dish coated with cooking spray. Sprinkle with cheese. Bake at 375° for 50-60 minutes or until a knife inserted in the center comes out clean.