Sauté sliced leek and onion and crushed garlic in olive oil until transparent; then add tomatoes (or tomato concentrate), saffron, bouquet garni, salt and freshly ground black pepper, to taste, and the fish for soup (any bony fish, or cod, whitefish, etc. and fish bones and heads). Add water to cover and cook for 30 minutes.
Then pass this very thick soupe de poissons through a fine sieve. Pour it into a clean saucepan and cook the fillets of daurade or sea bass in it until tender. Remove fillets from soup; drain; arrange on croûtons in a large shallow soup bowl. Whisk Aïoli into soup over a gentle heat until well blended. Do not boil. Serve with Rouille.