Lettuce performs surprisingly well in soup. Use Boston lettuce or another leaf lettuce (not iceberg), and cook over very low heat until shreds have broken down in the broth. For a rich soup cut stock back to 3 cups and add 1 cup cream at the end of cooking time.
Combine lettuce with 1 cup stock in soup kettle. Cover and simmer over very low heat 15 minutes. Add another cup of stock and simmer 15 minutes longer. Repeat with remaining stock, cooking up to 1 hour, until mixture is smooth. Garnish with croutons.