Shred lettuce; divide in half. Place one half of shredded lettuce, chicken stock, onion, green pepper, and carrot in a large Dutch oven, reserving remaining half of lettuce. Bring to a boil. Reduce heat; cover and simmer 30 to 40 minutes. Remove from heat, and let cool. Strain, reserving stock in Dutch oven; discard vegetables.
Add salt, pepper, and reserved uncooked lettuce to stock. Cover and simmer 30 minutes.
Combine flour and water; stir well to make a paste, and add to soup, stirring well. Cook 5 minutes. Serve warm in soup bowls with cheese crackers, if desired.