In a large saucepan, combine the shrimp shells, quartered onion, celery, carrot, if using, parsley, salt to taste, and water. Bring to a boil, then simmer gently, with the lid slightly askew, for 15 to 20 minutes. Strain through a fine-mesh sieve. Discard the shells and vegetables. When you are ready to prepare the risotto, warm the broth to a gentle simmer.

Warm 2 tablespoons of the olive oil in a large high-sided sauté pan set over medium heat. Add the garlic and sauté lightly but do not brown, about 1 minute. Add the shrimp and cook until they turn light pink, about 4 minutes. Remove the shrimp from the pan to a plate and set aside.

Add the remaining 2 tablespoons olive oil, 1 tablespoon of the butter, and the chopped onions to the sauté pan. Cook over medium-high heat until the onions have softened but not browned, about 5 minutes. Add the rice, stirring to coat it with the olive oil. Stir in 1 cup of the simmering broth. When the rice has absorbed the broth, add the wine and tomato and stir until the wine has been absorbed by the rice. Add the remaining broth, ½ cup at a time, stirring frequently and allowing the rice to absorb the liquid after each addition, until the rice is al dente, 15 to 20 minutes. Add the reserved shrimp along with the last ladleful of broth and any juices that may have accumulated on the plate.

The rice should have a slight resistance to the bite. If it seems too hard, add a little more liquid and continue cooking for another minute or two. Remove the rice from the heat and briskly stir in the remaining 1 tablespoon of butter and the Parmesan, if using. Serve at once.