Preheat oven to 400 degrees. Place squash, cut side down, in a large baking dish. Place the carrots, celery, onion and garlic around the squash. Add 1 cup of water and place in the oven. Roast for 1 and 15 minutes or until the squash is very tender.
Let the vegetables cool about 20 minutes then peel the squash.
Puree the vegetables in a blender or in a large pot using an immersion blender. If using a blender you may need to do this in 2-3 batches.
Pour blended vegetables in a soup pot. Add the salt, thyme, pepper and broth. Simmer until everything is heated through, about another 10 minutes. You may want to add more broth depending on whether you prefer a thicker or thinner soup.