In a skillet cook bacon until crisp and drain, reserving 1 1/2 tablespoons fat. Crumble bacon. In a saucepan cook onion, leek, garlic, and bay leaf with salt and pepper in reserved fat, stirring, until softened. Add squash, apple, broth, and 1/2 cup water. Simmer mixture, covered, until squash is tender, about 15 minutes, and discard bay leaf. In a blender purée mixture and add enough additional water to thin soup to desired consistency. Whisk in sour cream and salt and pepper to taste and heat soup over moderately low heat until hot (do not boil). Serve soup topped with crumbled bacon, apple and sour cream.