This is a very simple soup but simply delicious and satisfying. For an even easier variation, use frozen, already-cooked squash.
Preheat the oven to 350°F.
Cover the cashews in water and soak them for several hours. This step is optional, but it helps them blend better.
Cut the pumpkin in half, remove the seeds, and bake cut-side down until very tender (45 minutes to 1 hour). When the pumpkin has cooled, scrape the flesh from the peel (hold the pumpkin over a bowl while doing this to keep any accumulated juices) and puree in a blender in batches. Put the pureed pumpkin into a large soup pot.
Drain the cashews, then blend them in a blender until smooth and add them to the pumpkin puree. Rinse the blender with a little water and pour it into the pot. Add a little more water if the pumpkin-cashew mixture is too thick. Add ginger and salt to taste, and heat gently for a few minutes to blend the flavors.