Fry bacon in bottom of large dutch oven or stock pot, until crisp. Remove bacon, drain on paper towels, crumble and set aside.
Pour off all but 4 tblsp of the bacon drippings. Add onions, garlic and celery and saute on med-low heat until onion is translucent. being careful not to burn the garlic.Add chicken broth, bring to a boil, reduce heat, and simmer for about 20 minutes, or until vegetables are tender. Strain vegetables, RESERVING the broth.and puree veggies in blender or food processor. Return pureed veggies to the broth and add the milk, celery seeds and thyme. Return to a boil and cook 5 more minutes. Add salt and pepper to taste. Pour into bowls and top with bacon crumbles, cheese and the optional, chopped celery leaves, if using.
Note: I pulse my veggies in my food processor, because I like it a little chunky, and find this soup better the next day.