Preheat your oven to 400°F (200°C). Spread the cubed butternut squash and chopped carrots evenly on a baking sheet. Drizzle with olive oil and toss to coat, then roast until the vegetables are tender and caramelized around the edges, about 30 minutes.
While the vegetables roast, sauté the chopped onion in a large pot over medium heat until translucent and fragrant, about 5 minutes. Add the minced garlic and cook for another minute until aromatic.
Remove the roasted vegetables from the oven and add them to the pot with the sautéed onion and garlic. Pour in the vegetable broth and bring the mixture to a gentle simmer.
Use an immersion blender to blend the soup directly in the pot until completely smooth and velvety. Alternatively, carefully transfer the mixture to a blender in batches, then return it to the pot.
Season the soup with salt and pepper to taste. Let it simmer for an additional 5 minutes so the flavors meld together.
Serve the soup hot, garnished with a drizzle of olive oil or fresh herbs if desired, and enjoy its warm, comforting texture.
