In a large pot over medium heat, warm the olive oil. Add chopped onion and celery and sauté for about 2 minutes, until they start to soften.
Add the rosemary sprigs, chopped carrots and squash. Stir for 2–3 minutes, allowing vegetables to mingle with the aromatics.
Pour in the vegetable broth, bring to a gentle boil, then reduce the heat. Cover and simmer for about 30 minutes, or until carrots and squash are very tender.
Remove the rosemary sprigs. Use an immersion blender (or carefully transfer to a blender in batches) to blend the soup until completely smooth and creamy.
Return the soup to the pot (if needed), taste, and season with salt and plenty of freshly ground black pepper. Warm through if necessary.
Ladle into bowls. If you like, drizzle a little olive oil on top and sprinkle with toasted pumpkin seeds or croutons for a bit of crunch.