Cook bacon in a large saucepan over medium heat until crisp. Remove bacon from pan, reserving 1 tablespoon drippings in pan. Crumble bacon; set aside. Add leek to drippings in pan; cook 7 minutes or until tender, stirring frequently. Stir in beans, broth, and water; bring to a boil. Reduce heat, and simmer 10 minutes or until beans are tender.
Place half of the bean mixture in a blender. Remove center piece of blender lid (to allow steam to escape); secure lid on blender. Place a clean dishtowel over opening in blender lid (to prevent spills), and process until smooth. Pour pureed bean mixture into a large bowl. Repeat procedure with remaining bean mixture. Stir in lemon juice, salt, and pepper. Ladle about 1 cup soup into each of 8 bowls; top each serving with 1 tablespoon onions, 1 tablespoon sour cream, and about 1 teaspoon bacon.