- In a large pot over medium-high heat, soften the onions in the butter. Add the broth and potato and bring to a boil. Simmer for 20 minutes or until the potato is tender.
- Add the peas and lettuce and bring to a boil. Simmer for 2 minutes.
- In a blender, purée the soup until smooth. Season with salt and pepper. Strain and add the cream, if desired.
- Serve in bowls and garnish with the croutons and chervil.