Cut squash into 1-inch chunks. In large pot melt butter. Add onion and cook until translucent, about 8 minutes. Add squash, apple and stock. Bring to a simmer and cook until squash is tender, about 15 to 20 minutes. Use an immersion blender to cream into soup. Stir and season with nutmeg, salt, and pepper. Garnish with chives or black pepper and serve.