Melt butter in saucepan. Add onion and sauté for 5 minutes or until golden. Blend in flour and cook for 1 minute. Gradually add 2 cups water and cook, stirring until thickened a bit. Add potatoes, carrots, celery and seasonings. Cover and simmer 20 minutes or until all veggie are tender. Add tuna, including liquid, and milk. Stir gently and heat trough. Season again to taste. Serve!