Melt the butter in a large heavy pot over low heat. Add the onions and curry powder and cook, covered, until the onions are tender, about 25 minutes.
Meanwhile, peel the squash (a regular vegetable peeler works best). Cut in half horizontally, scrape out the seeds, and chop the flesh.
When the onions are tender, pour in the stock, add the squash and chopped apples, and bring to a boil. Reduce the heat and simmer, partially covered, until the squash and apples are very tender, about 25 minutes.
Pour the soup through a strainer, reserving the liquid, and transfer the solids to a food processor, or use a food mill fitted with a medium disc. Add 1 cup of the cooking stock and process until smooth.
Return the puréed soup to the pot and add the apple juice and about 2 cups more stock, until the soup is of the desired consistency.
Season with salt and pepper, simmer briefly to heat through, and serve immediately, garnished with the shredded apple.