In a large saucepan, saute celery and onion in butter until tender. Add the broccoli, broth, garlic salt and pepper; bring to a boil. Reduce heat; cover and simmer for 45 minutes or until broccoli is tender. , In a small bowl, combine cornstarch and water until smooth. Stir into soup. Bring to a boil; cook and stir for 2 minutes or until thickened. Reduce heat to low. Stir in cream; cook 10 minutes longer or until heated through.