Peel your butternut squash and then scoop the seeds out with a spoon. Cut the squash into cubes no larger than an inch.
Quarter your onion and leave the garlic whole. Add everything to a sheet pan (I line mine with parchment paper for easy clean-up) and drizzle with the olive oil, 1 teaspoon salt, and ½ teaspoon pepper. Toss everything to combine.
Roast in the oven for 45 minutes.
Add your roasted vegetables, the cream, 1 teaspoon salt, and the parmesan cheese to the blender and blend on high for a few pulses.
Add your water ½ cup at a time, blending in-between each addition, until your sauce reaches your desired thickness.
Notes
NOTES:If you bought a huge squash and are having trouble slicing into it, poke it all around with a fork and microwave it for about 5 minutes. It will be a little bit softer and easier to cut. BE CAREFUL WITH THE BLENDER! Steam from the hot veggies can cause your blender lid to pop off. To avoid this just open the little vent on the top of your blender lid and cover loosely with a towel to prevent splatters. Because there are so many different variables in this recipe (size of the squash, size of the onion, whether you used cream or milk...) add your water in little by little. If you dump it all in at once, it may be more than you needed and ruin your sauce.