Heat olive oil, minced onion, and minced garlic in a large frying pan over medium heat, stirring often, for 3 minutes.

Add the fish fillets and cook over low heat for 4 minutes, stirring occasionally.

Set the fish aside.

Put the clams and mussels into a large pot with water and white wine.

Cook, covered, over medium-high heat until the shells have opened, stirring occasionally.

Remove the mussels and clams from their shells and add to the pan with the fish.

Strain the juices from the pan the clams and mussels cooked in through a fine strainer lined with a double layer of cheesecloth into a saucepan.

Add water to the seafood broth and bring it to a boil.

Reduce heat to a simmer.

Heat olive oil in a heavy-bottomed saucepan.

Add the rice and stir constantly for 2 minutes.

Ladle in seafood broth and, stirring vigorously and constantly, cook until the liquid has been absorbed.

Add another ladleful of broth and stir the rice until the liquid has been absorbed.

Add the cooked seafood, shrimp, tomatoes, parsley, cayenne pepper, if using, and broth to the risotto.

Stir and continue adding the broth, a little at a time, stirring until the liquid has been absorbed and the risotto is done.

If necessary, add boiling water to the seafood broth and keep it simmering.

Season the cooked risotto to taste with salt and pepper and stir in the butter.

Turn the risotto into a warmed serving dish or bowl and garnish with the parsley sprigs.

Serve immediately.