Heat oven to 375° F. Peel the squash with a vegetable peeler, scoop out seeds, and cut into 1/2 to 1-inch cubes. Toss the squash cubes with the chopped onion, 1 scant teaspoon of salt, a dash of pepper and the 2 tablespoons of olive oil. Spread out in a large baking pan and bake for 30 minutes, until tender and lightly browned.
Heat 1 tablespoon of butter and 1 tablespoon of oil in a medium saucepan over medium-low heat. Add celery and sage and cook, stirring, until celery is just tender. Add chicken broth and the roasted vegetables. Bring to a boil. Reduce heat to low, cover and continue cooking for about 30 minutes, until vegetables are very tender. Carefully blend in small batches. Taste and add salt and pepper. Serve your buttercup squash soup topped with grated Parmesan or Greek yogurt.