1. Preheat oven to 350 degrees Fahrenheit.
2. In a large frying pan, saute onions until golden. Add celery and mushrooms and continue to saute until vegetables are softened.
3. Stir in flour and mix until combined. Add water, onion soup mix and salt, to taste, stirring until mixture thickens.
4. Sprinkle in half a cup of shredded mozzarella cheese and continue to stir until cheese is melted. Remove from heat.