Preheat oven to 425 degrees. Place squash and onion in a large baking dish. Pour in chicken broth and bake, uncovered, stirring occasionally, about 1 hour. (It's OK if some of the pieces get browned, this will add to the roasted flavor of the soup.).
Remove from oven and let the vegetables cool for about 15 minutes; puree in batches in a blender or food processor.
In a small skillet, toast curry powder over low heat until fragrant, about 2 minutes; stir into pureed soup. Season to taste with salt and pepper; garnish with sour cream and scallions.