In a heavy saucepan over medium heat melt butter and saute the onion until tender. Pour in chicken stock and water. Mix in celery, carrots and potatoes then cook 10 minutes stirring occasionally until vegetables are tender.
In a small bowl whisk together 1 1/2 cups (375ml) milk and flour. Mix into the saucepan.
Mix remaining milk, celery salt, salt and pepper into the saucepan. Stirring occasionally continue cooking the mixture about 10 minutes until thickened.
Stir fish into the mixture and cook 5 minutes or until fish is easily flaked with a fork.
Mix in cheddar cheese and cook another 5 minutes until melted.