On the day before serving, prepare the cooking liquid: Soften the saffron threads in a small bowl of hot water. In a large Dutch oven or heavy soup pot, on top of the stove, sauté the onions, leeks, celery and fennel in olive oil until soft, but not brown. Add the tomato sauce, garlic and bouquet garni. Stir in the fish stock, the saffron and liquid, and season to taste with salt and pepper. Bring the mixture to a boil and simmer for 30 to 40 minutes. Refrigerate the liquid for cooking the seafood.