How to Make It
Melt the butter in a large saucepan. Add the leeks, onions and 2 teaspoons of salt and cook over moderate heat, stirring occasionally, until the vegetables are softened, about 10 minutes. Stir in the vermouth and white wine and boil until nearly evaporated, about 5 minutes. Add the potatoes and water and bring to a boil. Simmer over moderate heat until the potatoes are very tender, about 35 minutes.
Working in batches, puree the soup in a blender until very smooth. Return the soup to the pan. Add the crème fraîche and bring to just a simmer over moderate heat, stirring occasionally. Season with salt. Ladle the soup into heatproof glasses or bowls. Garnish with the chives and pepper and serve.