Peel tomatoes and remove seeds. Cook tomatoes and onions slowly in olive oil with a small bouquet garni. Wash and clean mussels. In a pot, cover mussels with 1 liter of cold water and simmer over moderate heat. Remove opened mussels and strain the mussel broth through a fine sieve. Add broth to the tomatoes, bring to a boil and simmer for about 10 minutes. Remove bouquet garni, adjust salt, add saffron and vermicelli broken into small pieces. Stir and let simmer until vermicelli are cooked al dente. Shell mussels and add to soup, letting it rest for about 2 minutes. Present the soup in a tureen accompanied with slices of toasted baguette and grated cheese.