Cut squash into 1-inch chunks. In large pot melt butter. Add onion and cook until translucent, about 8 minutes. Add squash and stock. Bring to a simmer and cook until squash is tender, about 15 to 20 minutes. Remove squash chunks with slotted spoon and place in a blender and puree. Return blended squash to pot. Stir and season with nutmeg, salt, and pepper. Serve.