Melt butter in a large heavy-bottom pot.
Add onion and cook, stirring until softened and translucent.
Add broth, season with salt and pepper and bring to a boil.
Stir in peas and lettuce.
Lower heat to simmer and cook for 10 minutes.
Allow soup to cool.
Puree soup with immersion blender until smooth.
Return to pot and re-heat gently.
Add a dollop of creme fraiche and serve immediately.