Position a rack in the middle of the oven and preheat to 450 degrees.
Line a jelly-roll pan with foil.
Halve the squash lengthwise.
Using a sharp metal spoon, scrape out and discard all the seeds and fibers.
Lay the squash halves, cut sides down, on the jelly roll pan and bake them until they are very tender when pierced with a fork, about 40 minutes.
Remove the pan from the oven and set aside to cool.
When squash is cool enough to handle, scoop the flesh, including any well-browned edges, out of the peels.
Chop and reserve the flesh.
Melt the butter in a large pot over moderate heat. Add the apple, onion, ginger and garlic and cook uncovered, stirring once or twice until lightly browned, 15 to 20 minutes.
Add vegetable stock, reserved squash flesh, salt and pepper and bring to a simmer.
Partially cover the pan and cook the soup, stirring occasionally, until the apples are tender, 30-35 minutes.
Remove from heat and cool slightly.
Puree the soup in a food processor or blender.
Serve with Ginger Cream and sage, or cool, cover and refrigerate.