Put leeks, onion, butternut squash, sweet potato, carrots, celery, and apple in a 5+ quart slow cooker. Pour vegetable or chicken stock over the veggies and season with salt, nutmeg, and pepper.
Cook on low 6 to 8 hours until vegetables are tender.
Puree using an immersion blender, or transfer in batches to a blender. Blend until smooth. Stir in heavy cream and serve hot. Freeze any leftovers for later winter enjoyment.