Preheat oven to 400° F. In a large bowl, toss the squash with 2 tablespoons of the olive oil, 2 teaspoons of the salt, and the pepper. Place the squash on a rimmed baking sheet and roast in oven for 15 minutes. Turn the cubes over and continue roasting for 15 minutes or until they are caramelized; set aside.
In a Dutch oven or a large stockpot, heat the butter and the remaining oil over medium heat. Add the onion, celery, and sage and sauté, stirring occasionally, until the vegetables are translucent and tender, 10 minutes. Add the squash, broth, and the remaining salt and bring to a boil. Lower heat and simmer for 30 minutes or until the liquid is flavorful. Remove from heat.
Using a blender or a food processor, blend the soup in batches until smooth. Return to the pot and keep warm. Top with Sage and Parmesan Croutons and the grated Parmesan.