Bring stock, pint of tomatoes, celery and onion to a boil. Reduce heat, cover and simmer until vegetables are tender, about 20 minutes. Puree mixture and set aside. Add fresh tomatoes and butter to pan; saute 5 minutes. Stir in flour until smooth. Add Purity Half & Half and cook over low heat until thickened. Stir in vegetable puree and sherry, if desired.