Heat the oil and butter in a heavy soup pot set over medium heat. Add the onion and salt and saute for 2 minutes and then cover and let sweat until translucent, about 3 minutes. Uncover and cook, stirring occasionally, until the onions begin to brown, 3 to 5 minutes. Add the cumin and coriander and cook, stirring, until very fragrant, about 30 seconds. Stir in the tomatoes and their juices, season with a few grinds of pepper, and cook for 2 minutes. Cover and simmer for another 10 minutes. Peel the roasted squash and add the flesh to the pot, breaking it up with a wooden spoon. Add the chicken broth, cover, and bring to a simmer. Adjust the heat to maintain a simmer and cook, covered, for 30 minutes. Let cool slightly and puree in small batches in a blender or a food processor (don't fill the vessel more than one-third full and vent the lid or you risk getting splashed with hot soup). Taste and add more salt and pepper if needed. Return the soup to the pot and stir occasionally over low heat until hot. Garnish each serving with a spoonful of yogurt or a drizzle of cream and a sprinkling of the minced parsley.