Preheat oven to 400° and line a rimmed baking sheet with parchment paper.
Half the squash and remove the seeds.
Place squash on the pan and drizzle each half with just enough olive oil to lightly coat the squash on the inside (about ½ tsp each).
Rub the oil over the inside of the squash and sprinkle it with salt and pepper.
In a plastic bag or bowl put the onions, carrots and garlic.
Add about 1 tablespoon of olive oil and toss until the veggies are coated.
Turn the squash face down, spread the onions, carrots and garlic around the squash and roast until it is tender and completely cooked through (about 40-50 minutes).
Set the squash aside until it is cool enough to handle, about 10 minutes.
In a large pot add 4-5 cups of the vegetable broth and veggies.
Use a large spoon to scoop the butternut squash flesh into the pot.
Discard tough skin.
Add the maple syrup, nutmeg and a few twists of freshly ground pepper and salt to taste.
With an immersion blender, carefully blend the soup.
Add more broth as needed to get to the consistency you want then add 1-2 tablespoons of butter to taste and blend again.
Chef's notes: Don't worry if the skin and flesh brown during roasting—that's good for flavor. Taste the soup before adding salt, as the vegetable broth will probably be a little salty so you won't need much.