Melt butter in a large pot over medium heat. Stir in leeks and onions; cook and stir until onion softens and turns translucent, about 5 minutes. Add potato, squash, carrots, apple, and chicken stock. Bring to a boil, then reduce heat to medium-low, cover, and simmer until vegetables are soft, about 20 minutes.

Carefully purée soup in batches in a blender, or use a stick blender to purée soup in the pot. Once soup is puréed, return it to the pot and stir in wine and cream. Season with nutmeg, salt, and pepper; simmer gently for 5 minutes. Ladle into bowls and garnish with chopped chives.