How to Make It
Melt the butter in a medium sauce-pan. Add the leeks and garlic and cook over moderate heat, stirring often, until lightly browned, about 10 minutes. Add the stock; season with salt. Cover and simmer over low heat for 15 minutes.
Stir in the lettuce and cook over moderate heat for 5 minutes. Stir in the tarragon. Puree half of the soup at a time in a blender until smooth. Transfer to a medium saucepan and gently reheat. Season the soup with salt and pepper and serve in warmed bowls.