Cook the onions and leeks slowly in olive oil for 5 minutes or until tender but not brown.Stir in the garlic and tomatoes. Raise heat to moderate and cook 5 minutes more.Add the seafood stock, herbs, seasonings and shellfish shells and cook uncovered at a moderate boil for 30-40 minutes.Strain the soup though a mesh sieve, pressing juices out of ingredients. You should have 2 1/2 quarts liquid, if not add additional seafood stock or clam juice.Correct the seasoning by adding salt or a bit more saffron if you feel it necessary.Take your 2 1/2 quarts liquid and place in a tall stockpot.Bring the soup to a rapid boil 20 minutes before serving. Add lobster, crab, and fish. Quickly bring back to a boil and boil rapidly for 5 minutes. Add the shrimp, clams and mussels and simmer for 5 minutes longer or until the fish are just tender. Do not over cook or seafood and fish will become tough.Serve immediately with a sprinkling of fresh parsley on top. The traditional French way of serving this aromatic soup is to ladle the broth into bowls and serve the fish and seafood on a separate platter.