3-4cups(32 oz) low sodium vegetable or chicken broth (see note)
Heat oven to 400ºF. Peel and seed the squash. Cut into 1-inch cubes and place them on a rimmed baking sheet. Drizzle with 1 tablespoon olive oil, salt, and pepper; toss to coat. Spread in a single layer and roast for 15 minutes. Turn and continue to roast until softened, about 20-25 minutes more.
Meanwhile, heat the remaining 1 tablespoon oil in a stockpot or Dutch oven over medium heat. Add onion and celery, and cook until softened, about 7 minutes. Add sage and saute 1 minute more.
Add broth and roasted squash and bring to a boil. Reduce heat and simmer for 30 minutes, or until vegetables are very tender. Remove from heat. Let cool for about 10 minutes. Use a handheld (immersion) blender to blend soup directly in the pan. Return to stove and warm over medium-low heat. Add broth as needed to thin as desired. I typically add about 1 cup.
Taste and add salt and pepper as needed.
I love to serve this with crumbled cooked bacon if I'm not serving it to vegetarians or vegans. It's also great with homemade croutons.
To save time, you could skip the roasting step and cook the squash cubes in the broth until tender. It won't have as much flavor, but it will reduce cooking time slightly.
If desired, you can roast the squash a day or two ahead and refrigerate until ready to make soup.