Cook bacon in a 6-quart heavy pot over medium heat, stirring occasionally, until crisp, then transfer with a slotted spoon to paper towels to drain, reserving 2 tablespoons fat in pot. (Add vegetable oil if your bacon is very lean and doesn't yield enough fat.)
Cook celery, carrot, and onion in fat in pot over low heat, covered, stirring occasionally, until softened but not browned, 10 to 12 minutes. Add cinnamon and cook, uncovered, stirring, 1 minute.
While vegetables are cooking, peel potatoes and coarsely chop. Peel and core 2 apples and coarsely chop.
Stir squash, potato, chopped apple, stock, 2 cups water, 1 teaspoon salt and ½ teaspoon pepper into onion mixture, then simmer, uncovered, stirring occasionally, until vegetables are very tender, 15 to 20 minutes.
Puree soup, add water and cream until consistency is good
Cut enough remaining apple into thin matchsticks (about 1½ inches long) to measure about ½ cup.
Serve soup topped with sour cream (if using), bacon, and apple matchsticks.
