First prepare the fish stock, cleaning the shrimp and putting the heads and tails in a pot together with the chopped onion, chilli, white wine and parsley: add a litre of water and bring to the boil, letting it cook for half an hour, then strain the fish broth and set it aside.
Open the mussels and clams in a pan with a drizzle of oil: once opened, keep them aside and add the filtered cooking water to the fish stock, bringing it back to the boil; you must have about one and a half litres of fish broth ready.
Remove the shells from the mussels and clams, keeping a few aside for decoration.
Clean the fish by removing skin and bones, cut the fish and cuttlefish into strips, taking care to remove all the bones with tweezers.
Stir-fry the prawns and set aside.
Prepare the risotto base with the chopped onion, a clove of garlic and a pinch of chilli heated in olive oil: remove the garlic when it becomes coloured and add the rice, delicately toasting it. Deglaze the rice with a little white wine, add the fish and cuttlefish with a little fish stock. Cook over low heat for 8 minutes, continuing to add the broth. Add the shrimp tails you kept aside and continue to cook, stirring. One minute before removing from the heat, add the mussels and clams and turn off the heat, letting your seafood risotto rest for a minute.
Serve garnished with a sprinkle of parsley and sauteed prawns.