If using fresh squash, peel, seed and cut in large chunks. If using frozen squash puree, let thaw. In a large saucepan, boil onion and fresh squash in lightly salted water, until just tender, or just boil onions till tender if using frozen squash puree. Drain well, then (quickly) puree, either with an immersion blender or by batches in a stand blender, together with butter, cream, maple syrup, apple cider, cinnamon and nutmeg. Add salt and pepper to taste. Heat thoroughly, and serve.