Bring a large pot of water to a boil. Meanwhile, slice the mozzarella into thin slices and place them in a colander or on a piece of kitchen paper to drain. Set aside.

Once the water boils, add a generous amount of salt, then add the pasta and cook in the rapidly boiling water. After half the cooking time specified on the package, drain the pasta in a colander, toss with a little olive oil, and let it steam dry.

Meanwhile, preheat the oven to 180°C (350°F). Place the spinach and tuna in a fine-mesh sieve and press firmly with a spoon to extract as much water as possible.

Place the spinach, tuna, ricotta, egg, and Parmesan cheese in a bowl. Finely grate the zest of the entire lemon and add it to the bowl. Season generously with salt, pepper, and chili flakes, and mix well until a homogeneous mixture is formed.

Pour the tomato sauce into the casserole dish. Divide the water between the two jars, swirl well, and pour into the casserole dish.

Now fill the shell pasta one at a time with the spinach and ricotta mixture and place them side by side (with the opening facing up) in the tomato sauce. Once all the pasta is filled and arranged in the tomato sauce, cover the baking dish with aluminum foil and bake in the oven at 180°C for 15 minutes.

After 15 minutes, remove the pasta from the oven, remove the aluminum foil, distribute the cheese over the pasta and return it to the oven for another 20 minutes or until a golden-brown cheese crust has formed on the pasta and the pasta is al dente.

Remove the pasta from the oven and let it cool for about 3 minutes (don't worry, it will be hot enough). Then top with fresh basil, if desired, and serve.