First of all soak the clams in fresh water for at least 2 hours, to purge them from the sand.
In the meantime proceed with cleaning the mussels: remove the filament, scrape the dirtiest shells with a knife or a metal wool pad, then rub them together under running water, 2 at a time.
After 2 hours, rinse the clams well to remove any sand residues.
Take 2 large pans, put the mussels in one and the clams in the other, cover with a lid and cook over high heat for a few minutes to open.
Then shell the seafood (just leave someone in the shell to decorate the dish).
Strain the cooking liquid with a narrow mesh strainer and set it aside.
Wash and peel the potatoes, then cut them into cubes.
Fry the chopped onion in oil in a saucepan, then add the potatoes and let them cook for a few minutes.
Add the tomato, basil and the cooking liquid of the seafood, then add water to completely cover the potatoes.
Cover with a lid and bring to a boil.
After 10-15 minutes from boiling, add the pasta, add a little salt and water if necessary, remove the lid and continue cooking over medium-low heat.
When the pasta is almost cooked, add mussels, clams, shrimp and parsley.
When the pasta is al dente, add the pecorino and season with salt if necessary.
Serve the pasta and sea potatoes, decorated with seafood kept aside with the shell, add a little pepper and serve.
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