Heat the oil and butter in a large Dutch-oven pot.
Add the onion, garlic, green pepper, and mushrooms. Sauté for 10 minutes or until the onion is limp.
Add the tomatoes, tomato paste, wine, clam juice, lemon juice, parsley, bay leaves, oregano, basil, salt, pepper, and sugar. Heat to a boil.
Reduce the heat and cover. Simmer for 1 hour, stirring occasionally.
Add the fish. Cook for 10 to 15 minutes or until the fish is flaky.
Add the clams (or mussels), shrimp, scallops, and crabmeat. Cover and simmer on low heat until clams (or mussels) open. Discard any clams that do not open.
Serve in large, flat soup bowls.