1. In a large saucepan over medium, sauté the onion, celery, thyme, rosemary, and sage in the olive oil for 5-7 minutes, until the vegetables turn tender. Add the salt and pepper and sauté the vegetables and herbs for an additional 2 minutes.
2. Add the butternut squash and broth to the pan and bring the soup to simmer (add enough water to just cover). Reduce the heat slightly, cover, and simmer the soup for 20 minutes, until the squash is tender.
3. Puree the soup in a countertop blender or with an immersion blender until it is completely smooth. Stir in the heavy cream and heat through. Serve the bisque hot, garnished with croutons.