To prepare soup, melt butter in a large Dutch oven over medium heat. Add leek and shallots; cook 5 minutes or until tender, stirring frequently. Stir in celeriac and next 5 ingredients (through bay leaves); bring to a boil. Cover, reduce heat, and simmer 15 minutes or until vegetables are tender. Stir in celery, milk, salt, and pepper; simmer 10 minutes (do not boil). Remove from heat; let stand 5 minutes. Discard bay leaves and thyme.
Place half of celery mixture in a blender; process until smooth. Pour pureed celery mixture into a large bowl. Repeat procedure with remaining celery mixture. Stir in half-and-half.
To prepare croutons, arrange bread in a single layer on a baking sheet; coat with cooking spray. Broil 1 minute or until golden. Turn bread over; sprinkle each bread slice with 1 tablespoon cheese. Broil 1 minute or until cheese melts; cool 1 minute on baking sheet. Place bread on a cutting board; cut each bread slice into 6 wedges to form croutons. Ladle about 1 cup soup into each of 8 bowls; top each serving with 6 croutons. Serve immediately.