Soak cauliflower, head down, one hour in cold water to cover; cook in boiling salted water twenty minutes. Reserve one-half flowerets, and rub remaining cauliflower through sieve. Cook onion, celery, and bay leaf in butter five minutes. Remove bay leaf, then add flour, and stir into hot stock; add cauliflower and milk. Season with salt and pepper; then strain, add flowerets, and reheat.