Cream of Leek and Potato Soup

4 Tbsp unsalted butter
3 medium leeks, white and lt. green parts only, thinly sliced crosswise
2 medium onions, halved and thinly sliced
kosher salt
¼ cup dry vermouth
¼ cup dry white wine
1 lb potatoes, peeled and cut into 1″ cubes
8 cups water
½  cup crème fraîche or sour cream
1 tbsp finely chopped chives
freshly ground pepper

Melt the butter in a large saucepan. Add the leeks, onions, and 2 tsp 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 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 crème fraîche and bring just to a simmer over moderate heat, stirring occasionally. Season with salt. Ladle the soup into heatproof glasses or bowls. Garnish with the chives and pepper to serve.

DO AHEAD: The pureed soup can be refrigerated for up to  2 days. Add the crème fraîche and reheat just before serving.

Recommended wine for use: Full bodied, Minerally Chardonnay from Burgundy: 2011 Jean-Marc Brocard Vieilles Vignes Chablis.