In a large soup pot, or preferably large Dutch oven, over medium-low heat, heat olive oil, then add butter and let melt. Add onions, garlic, bell pepper, celery and parsley. Cook until soft, about 8 minutes.
Add tomatoes (break them up with your hands as you add them to the pot), and add the juice from the can.
Add the clam juice, wine, bay leaf, basil, oregano, thyme, red pepper flakes, salt, and lemon juice; bring to just a boil, then reduce heat and simmer for 1 hour.
NOTE: At this point, stock may be refrigerated, covered and kept up to 2 days before serving. Just bring the stock back to a boil about 20 minutes before serving, lower the heat, and progress with the recipe.
Scrub the clams and mussels with a small stiff brush under cold running water; remove beards from mussels.
Discard any open clams or mussels. Let cold water run over shells for 5 minutes (shaking every minute, or so).
Gently stir into the sauce the clams, mussels, shrimp, scallops, and fish fillets. Cover the dish and let simmer for 7 - 8 minutes, until the clams and mussels open. Discard any shells that have not opened.
Add the crab legs and steam until heated through, another 2 - 3 minutes.
Try not to stir too much (it will break up the fish), and don't overcook!!
Remove the bay leaves. Adjust seasonings
Ladle sauce and seafood into large bowls and serve with toasted bread.