In a large pot, sauté the onion, shallot and fennel gently is abundant olive oil until the onion is very soft and translucent, seasoning as you go with salt and black pepper. (Add a spoonful of water if you like, to help soften the vegetables and prevent browning.) Then add the garlic and red pepper, and continue the sautéing for just a few moments, until you can smell the garlic's aroma.
Add the tomato paste and continue cooking for about 30 seconds or so to 'open' its flavors. Then add the milled (or crushed) tomatoes. Allow the tomatoes to simmer for 10-15 minutes, or until they start to separate from the oil and take on a 'saucy' consistency. Taste the tomatoes; they should have lost their rawness—if not, continue cooking for a few more minutes. Then add the fish stock or clam juice, and the white wine, together with the thyme and bay leaf. Simmer for another 10-15 minutes to allow the flavors to meld and the liquid to reduce a bit. Taste the liquid carefully; it should be well seasoned and quite intense. If not, adjust for seasoning and, if it seems a little thin, let it simmer a bit more.
Now, add the seafood, beginning with the fish, then the shrimp (and scallops and crabs if using) then the clams and mussels on top. Bring up the heat to a brisk simmer, cover the pot, and let it all cook for about 5 minutes, or until the clams and mussels open. Top with some minced parsley and a drizzle of olive and serve.