To make the stock simmer the fish head and bones, the onions, and bouquet garni in 8 cups of water for 20 minutes. Strain. Heat the olive oil in a flameproof casserole or dutch oven and cook until softened, about 10 minutes, stirring occasionally. Stir in the tomatoes, half the white wine, the strained stock, and saffron. Simmer for 20 minutes. Bring the remaining white wine to a simmer in a large saucepan and add the mussels. Cover and cook for 5 minutes. Strain the mussels through a cheesecloth-lined sieve, reserving the cooking juices. Remove the mussels from their shells and add them to the stock, along with the cooking liquid. Stir in the crÅme fraöche and re-heat gently. Serve the soup with the grated cheese on the side.
