Preheat your oven to 425. While it is preheating, prep your butternut squash by chopping off the stem on top and then cutting your butternut squash in half lengthwise. Using a spoon, scoop out the seeds and stringy parts from the center of the squash and discard. Line a baking sheet with aluminum foil and place both halves of the squash face down on the sheet.
Once the oven is preheated, roast your squash in the oven for 45-50 minutes. When it's done it should be very soft and you should be able to easily pierce the flesh with a fork.
Set your squash to the side and allow it to cool a bit. While it is cooling, sauté your diced onion on the stove top on medium-high heat with a bit of water until fork tender (this typically takes me about 8 minutes). As it cooks, add small amounts of water if necessary to prevent the onion from sticking to the pan. Add your garlic to the pan and sauté for another minute. Remove from heat.
Once your squash is cool enough to handle, scoop out as much of the flesh as you can and add it to a blender. This process can sometimes be a bit messy depending on how soft your squash got! You want to separate the flesh from the skin as much as possible, but technically the roasted skin is edible - so don't stress if little bits of it sneak their way in! Discard the large chunks of skin once you are done.
Add your sautéd garlic and onions, 3 cups of the veggie broth, and all other ingredients to the blender. Blend until smooth, and add more veggie broth as needed to reach desired consistency. Add more seasonings to taste if necessary.
By now your soup is likely still warm from the roasted squash, but if you'd like it hotter just pour it into a soup pot and heat it on the stove until it reaches your desired temperature.
Serve with optional toppings such as roasted pumpkin seeds or a sprinkle of cinnamon. Enjoy!