Preheat oven to 400°F (200°C).

Cut butternut squash in half lengthwise and scoop out seeds.

Place squash cut-side up on a baking sheet and roast for 30-40 minutes, or until tender.

While squash is roasting, heat olive oil in a large pot over medium heat.

Add onion and cook until softened, about 5 minutes.

Add garlic and cook for 1 minute more.

Scoop out the flesh of the roasted butternut squash and add it to the pot.

Pour in vegetable broth, salt, and pepper.

Bring to a boil, then reduce heat and simmer for 10 minutes.

Use an immersion blender to blend the soup until smooth.

Serve hot.