In a medium-sized pot, heat oil.

Add onions and fry until soft and translucent.

Add celery and mushrooms and stir.

Add salt and half a cup water, cover, and bring mixture to a boil.

Add tuna and soy sauce.

Mix well and cook on low heat for 30 minutes.

In a small cup, combine cornstarch and two tablespoons water until smooth. Slowly pour into tuna mixture, stirring constantly until the mixture thickens.

Serve hot on a bed of rice.