For the spicy tomato sauce:. In a large saucepan, heat the oil over medium heat, then add the onions. Saute until soft and fragrant. Add the garlic and harissa and saute for 2 to 3 minutes. Add the tomatoes and bring to a boil. Stir in the broth and season with salt and pepper. Bring to a simmer, then simmer the sauce for 10 minutes before finishing with the parsley.