Preheat your oven to 400°F (200°C). Place halved butternut and acorn squashes face down on a lightly oiled sheet pan. Roast for about 50 minutes, or until the flesh is tender and easily pierced with a fork.
Let the squash cool slightly before scooping out the flesh and discarding the skins.
In a large Dutch oven or stockpot, heat the butter and olive oil (or avocado oil) over medium heat. Add the diced onion and cook for 3–5 minutes, stirring occasionally, until translucent and fragrant.
Add the roasted squash, vegetable broth, brown sugar, cream cheese, and salt to the pot with the onions. Stir gently to combine.
Use an immersion blender to puree the soup until silky smooth. If you don’t have an immersion blender, carefully transfer the soup in batches to a countertop blender, blending until no chunks remain.
Return the soup to the pot if using a countertop blender. Warm over medium heat for about 5 minutes, stirring occasionally. Ladle into bowls.
Optionally, garnish with croutons, pumpkin seeds, dried cranberries, or a swirl of heavy cream for presentation and extra flavor. Enjoy!