Preheat an oven to 400 degrees F. Place a large (about 12-cup) ovenproof dish over medium heat, about 1 minute. Add in the olive oil, the onion, garlic and sea salt. Saute until the onion is soft and translucent, about 5 minutes. Add the rice to the dish and stir for another minute or until the rice is lightly coated in oil and light gold in color.
Add the stock and the tomatoes to the dish. Bring the mixture to a simmer. Stir in the tuna, zucchini, and season with the black pepper, to taste. Cover the dish, with a lid or aluminum foil, and bake until the rice is tender and cooked through, about 30 minutes. Scatter the parsley over the top of the risotto and sprinkle with grated Parmesan cheese, if desired, before serving.