In a large stock pan, Saute the onion and celery until tender - stir often
Add the broth, salt, thyme and pepper
Bring the soup to a boil, and then reduce the heat to simmer
Allow to simmer, covered, for 20 minutes
Add the florets and cover again. Cook for 5 more minutes
Using a cup or bowl, Stir the milk and cornstarch together until smooth
Add the milk into the soup
Bring the soup back up to a boil, and continue to cook, stirring until thickened