Cook the pancetta in a 6-quart heavy pot over medium heat, stirring occasionally until crisp, then transfer the pancetta with a slotted spoon to a paper towel-lined plate. Reserve 2 tablespoons of fat in the pot. (Add vegetable oil to equal 2 tablespoons if your pancetta is very lean.)
Add the celery, carrots and onions to the pot, reduce the heat to low and cover the pot. Cook the vegetables, stirring occasionally, until softened but not brown, 10 to 12 minutes. Add the curry powder and cook, uncovered, stirring for 1 minute.
While the vegetables are cooking, peel and coarsely chop the potatoes. Peel, core and coarsely chop one apple.
Add the chopped potatoes, apple, butternut squash, stock, 2 cups water, 1 teaspoon salt and 1/4 teaspoon black pepper to the pot. Simmer the mixture, uncovered, stirring occasionally until the vegetables are very tender, 15 to 20 minutes.
Purée the soup in four batches in a blender until smooth. Return the puréed soup to the cleaned pot and thin it with an additional 1/2 cup of water, if necessary. Heat the soup over low heat until warmed throughout then season with additional salt and pepper, to taste.
Cut the remaining apple into matchsticks then spoon the soup into bowls and garnish with a dollop of sour cream, a few apple matchsticks and the reserved pancetta.
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