Preheat oven to 450°F. Line a baking sheet with parchment paper. Place the squash and onion on the prepared sheet and bake for an hour or longer, until the squash is completely tender when pierced through. (Baking time will vary depending on the size of the vegetables. If you use 2 smaller squash to total 3 pounds, they will cook quicker.) Remove squash and onion from oven and cut the squash lengthwise to accelerate cooling so you can handle. Meanwhile, add the water, cashews, lemon juice, rosemary, sea salt, cinnamon, allspice, and garlic to a blender. Puree until smooth and silky. Remove the skins and seeds from the squash and add the flesh to the blender. Remove the outer tough layer of skin from the onion and add the whole roasted onion to the blender. Puree with the cashew mixture until smooth. (I use my Blendtec. If your blender isn’t large, puree in batches.) If more water is needed, add enough to loosen/thin the mixture and puree again. Transfer the mixture to a pot, scraping the blender with a spatula to loosen all the mixture. Gently heat the soup, season with extra salt if desired, and extra water to thin if you like, then serve. Serves 4.