Melt the butter in a large pot over medium heat.
Add the onion and cook until softened (about 3-5 minutes).
Add the garlic and cook for 1 more minute or until fragrant.
Add the butternut squash, carrots, potatoes, celery, chicken broth, salt, pepper, nutmeg, and cayenne pepper. Bring the mixture to a boil, stirring occassionally.
Once boiling, reduce the heat to low. Cover the pot and simmer until the squash is fork-tender (about 20-25 minutes).
Use an immersion blender or a food processor to blend the soup until smooth. Be very careful as hot soup may splash.
Serve the soup warm, garnished with a light drizzle of heavy cream and black pepper.