Heat the butter and oil in a large pot. Mince the carrots, celery, onion, and garlic. Add to the pot with the dried herbs, salt, cayenne, and saffron and sauté over medium heat for five minutes. Drain and chop the tomatoes, add to the pot, and continue cooking for five minutes more. Add the mussels and wine, turn up the heat, and cover the pot until the mussels open.
Remove the mussels, lower the heat, and simmer for 30 minutes. Meanwhile, remove the mussels from their shells, cut any large mussels into smaller pieces and set aside until the soup is cooked.
Remove the solids from the pot, puree and return to the pot with the mussels, basil, and cream if desired. Reheat the soup just before serving, but if cream is used, do not allow it to boil.