How to Make It
Melt 4 tablespoons of the butter in a large, heavy saucepan. Add the shallots, celery, leek and potatoes, cover and cook over moderately low heat, stirring occasionally, until softened, about 8 minutes. Add the wine and lemon juice and cook over high heat until evaporated. Add 4 cups of the water and the bouquet garni and bring to a simmer. Cover and cook over moderately low heat until the vegetables are tender, about 15 minutes. Discard the bouquet garni.
Working in batches, puree the soup in a blender or food processor until smooth. Strain the soup into a clean saucepan through a fine sieve. Stir in the heavy cream.
Bring a medium saucepan of salted water to a boil. Add the nettles and cook just until wilted. Drain the nettles and immediately plunge them into a medium bowl of ice water. Transfer the nettles to the blender or food processor and puree them with the remaining 1/4 cup of water. Strain the nettle puree into the soup and season with salt and white pepper. Bring the soup to a simmer over low heat, stir in the remaining 2 tablespoons of butter until melted and serve the soup right away.