Begin by sautéing a few slightly crushed garlic cloves in abundant olive oil. If you like, you can add a bit of peperoncino to sauté in the olive oil along with the garlic.
When the garlic begins to give off its aroma and is just barely beginning to brown, add tomato. Season with salt and pepper (going light on the salt since the shellfish will be salty) and finely chopped parsley, then allow the tomatoes to simmer for about 10 minutes or so, or until they begin to reduce and reach a saucy consistency. Then add a splash of white wine.
Add your seafood, starting with the varieties that take the longest to cook, then progressively adding those that take less time: Begin with the mollusks. With very young calamaretti (baby cuttlefish) let them simmer about 10 minutes before you add any other fish. Octopus or mature squid (which you should cut up into bite-sized pieces) will take much longer, usually about 30 minutes.
Then add the fish and let that cook for another five to ten minutes, depending on the size.
Then add shrimp or scallops, if using, along with the clams and mussels and simmer them until they open.
Sprinkle with a bit more finely chopped parsley and serve immediately.