Preheat oven to 450 degrees F.
Spread squash on a large rimmed heavy duty baking sheet and toss with oil, salt, pepper and about half the rosemary. Roast at 450 for about 35 minutes, stopping halfway to turn, or until sides are nicely browned and you see some caramelization. The 22 oz of squash should cook down to about 10 oz.
Add broth to the blender, then add roasted buternut squash, onions, carrots, and everything else.
Close blender and select the "soup" cycle, using setting 2 for smooth. Hit start.
The blender will start to heat up and temperature will climb to 212 degrees. When it hits 212, a 12 minute countdown begins.
When the blender is finished, remove lid and add pepper to taste. If you've used unsalted broth, you'll probably need to add at least 1/4 teaspoon more salt. I use salted broth and end up adding an extra pinch or two.