This is based upon a recipe from The Cooking of Provincial France by Time Life books. My basis for the bouillon is always a mixture of whatever fish stocks I happen to have in the freezer at the time. The addition of some canned tomatoes makes up for the relatively low level of flavour in many fresh ones. If you have confidence in your fresh tomatoes, use more and omit the canned ones.
First, make the bouillon. (This can be done ahead of time.) Remove the meat from the crab, break up and reserve the shell. (Use the crab meat to make a starter!) Trim any whole fish being used and reserve the trimmings. Soften the onions and leeks in the olive oil for about five minutes without browning in a very large, heavy casserole. Add the fish stock, wine, tomatoes (both fresh and canned), herbs and seasonings. Add the reserved fish trimmings and the crab shell. Simmer uncovered over moderate heat for about an hour. Strain the bouillon through a chinois pressing well to extract every drop of flavour. Check the seasoning and add salt and pepper as necessary to taste.
Begin reheating the bouillon about 20 minutes before serving. When boiling, add the firmest and thickest of the fish and continue boiling for about 5 minutes. Then add the shellfish, prawns and any very soft or thin fish and boil for another 5 minutes.
To serve, remove the fish and shellfish from the soup and arrange them on a heated dish. Pour a ladle or two of the soup over the fish to keep it moist. Ladle large helpings of soup and fish into soup plates at the table. Aioli sauce is a must along with warm French bread and cold French white wine!