1. Cook bacon in a large skillet over moderate heat, turning, until crisp. Transfer to paper towels to drain; set aside at room temperature. Leaving 1 tablespoon bacon fat in skillet, add leeks, season with salt and pepper and cook over moderate heat, stirring, 2 minutes.
2. Transfer to slow cooker along with potatoes, thyme, and 2 cups water. Cook, covered, on high heat 1-1/2 hours. Remove thyme sprigs. Mash potatoes, then add remaining ingredients and 3 cups water. Cook on high heat 1 hour more. If a smoother texture is desired, blend soup, in batches, in a blender until smooth. Return each batch to slow cooker and reheat (if necessary) over low heat, stirring occasionally. Salt and pepper to taste. Ladle soup into bowls, garnish each with scallions and top with a slice of bacon. Makes 8 servings.