Preheat oven to 350 degrees Fahrenheit.

Place butternut squash halves face down in a baking dish filled with 1 inch of water.

Roast squash for 1 hour, or until tender.

In a large saucepan, sauté chopped onion and celery in 2 tablespoons of butter until tender but not browned.

Scrape the roasted butternut squash flesh from the skin and add it to the saucepan with the onion and celery.

Add the remaining 2 tablespoons of butter to the saucepan.

Mash the squash and vegetables together and season with salt and pepper to taste.

Pour chicken stock into the saucepan and bring to a simmer.

Add nutmeg.

Simmer for 20 minutes to allow the flavors to meld.

Blend the soup with an immersion blender until smooth.

Add half-and-half and heat through gently (do not boil).

Adjust salt and pepper to taste before serving.