Preheat the oven to 400°F (200°C). Spread the cubed butternut squash on a baking sheet, drizzle with a tablespoon of olive oil, and roast for about 25-30 minutes until tender and lightly caramelized.
While the squash roasts, heat the remaining olive oil in a large skillet over medium heat. Add the chopped onion and cook until translucent and fragrant, about 5 minutes.
Add the minced garlic, ground cinnamon, and ground sage to the onions, stirring constantly for about 30 seconds until the mixture becomes aromatic.
Transfer the roasted squash and sliced apples into the slow cooker, along with the sautéed onion and spice mixture. Pour in the vegetable broth and stir to combine everything evenly.
Set the slow cooker to low and cook for 4 hours, allowing the flavors to meld and the apples to soften.
Once cooked, use an immersion blender to puree the soup directly in the slow cooker until smooth and velvety. If using a regular blender, carefully blend the soup in batches and return to the slow cooker.
Taste the soup and season with salt and pepper as needed. Adjust the thickness by adding extra broth or reducing at high heat for a few minutes if desired.
Serve the soup warm, garnished with fresh herbs or a drizzle of cream if desired. Enjoy the rich, comforting flavors that embody the essence of fall.
