Preheat oven to 400 degrees. Peel and chop butternut squash and potatoes. Remove seeds from squash. Toss butternut squash and potatoes with 2 tablespoons olive oil and season generously with salt and pepper. Roast until tender, approximately 25 minutes. In a large pot over medium heat, melt butter and remaining tablespoon of olive oil. Add onion and cook until softened, approximately 6 minutes. Season generously with salt and pepper. Add roasted squash and potatoes and pour over chicken broth. Simmer 10 minutes, and then blend soup in blender until creamy. Serve with a dollop of sour cream. Serves 4.