Heat oil in large pot over medium heat. Salt and sauté fennel, onion, leeks, bell pepper and shallots until the onion is translucent, about 10 minutes.
Add garlic and red pepper flakes, saute 2 minutes.
Stir in and briefly sauté tomato paste.
Add tomatoes with their juices, wine, fish stock and bay leaf. Cover and bring to a boil. Reduce the heat to medium-low. Cover and simmer to blend flavors–about 30 minutes.
Add the clams, mussels and mushrooms. Cover and simmer until the shellfish begin to open; about 5 minutes.
Add the basil, shrimp, squid, scallops and fish. Simmer gently until the fish and shrimp are just cooked through, stirring gently, about 5 more minutes (discard any clams and mussels that have not opened by now). This would also be the time to add any crab or other crustaceans.
Season the stew to taste with more salt and red pepper flakes.
Serve the stew in bowls, garnish with parsley and serve.