In a large saucepan, heat the olive oil over medium heat.
Add the onion & sausage and cook over medium-high heat for 4-5 minutes, stirring occasionally.
Add the garlic and cook for 1 minute more. Add the wine and cook for 1 minute, scrapping the pan clean. Add the stock, Peperoncini Piccanti, tomato paste, tomato purée and pepper and bring just to a boil.
Cover partially and simmer over moderately low heat, stirring occasionally, 15-30 minutes.
Season the sauce with salt, pepper or Peperoncini Piccanti to taste.
Stir in the mussels and shrimp, raise heat to medium, cover and cook until the mussels open, 4 to 6 minutes; discard any mussels that do not open.
Sprinkle with onions & parsley and serve with lemon wedges and crusty bread.
