Sauté onion and garlic in olive oil for 2-3 minutes over moderately low heat, stirring occasionally. Add the rice and toast lightly for 1-2 minutes without letting the rice or onion take on any color.
Add the wine and cook until almost completely absorbed. Add most of the hot fish stock (it should come about ½ inch above the rice) and bring to a simmer. Cook the risotto, stirring occasionally, until the rice has absorbed some of the liquid, about 8 minutes.
Add the mussels, shrimp and squid (tentacles as well), and tomatoes, and continue cooking, stirring occasionally, until the seafood is cooked and the rice is al dente, about 6 minutes. As the rice continues to absorb the liquid, add more as needed.
Remove the risotto from the heat, add butter Grana Padano or Parmesan cheese. Stir well to release the starches and make the risotto creamy.