Preheat the oven to 425º and line a baking sheet with parchment paper. Slice acorn in half using a chef's knife but slicing it horizontally so that there is a top and a bottom. Keep the seeds inside the acorn squash while it roasts (it's easier to remove once it softens).
Dice a large onion and two carrots into large halves.
Place the acorn squash and vegetables on a baking sheet.
Drizzle olive oil and sprinkle cinnamon on the acorn squash and roast in the oven for 40-45 minutes or until the squash is fork tender.
Once the vegetables are done roasting, take out of the oven and let cool for 15 minutes. Scoop out the flesh of the acorn squash and add to a large blender, along with the other vegetables. Blend to combine the ingredients, then add in the stock and water. Blend again until very smooth.
Add the blended soup to a large stock pot and simmer on low for 20-30 minutes, seasoning with salt and pepper to taste.
Notes
Add a generous amount of oil to the vegetables as you bake them, this will help give them flavor and also help them render down as they roast.