In a skillet cook bacon until crisp and drain, reserving
1 1/2 tablespoons fat. Crumble bacon.
In a heavy saucepan cook onion, leek, garlic, and bay leaf
with salt and pepper to taste in reserved fat over moderate
heat, stirring, until softened.
Add squash, fresh herbs, apple, peeled and chopped. Stir
together, simmer a minute or two.
Add broth and
simmer mixture, covered, until squash is very tender, about
15 minutes, and discard bay leaf. Add milk (or cream) and
simmer slowly for 5 or 10 minutes more.
In a blender purée mixture in batches, transferring as
puréed to a clean saucepan, and add enough additional water
to thin soup to desired consistency.
Whisk in sour cream or crème fraîche and salt and pepper to
taste and heat soup over moderately low heat until hot (do
not boil).
