In a large pan, add the spaghetti, the green bell pepper chopped into cubes, the onion chopped into cubes, the tomato chopped into cubes, parsley to taste and the tomato sauce.
Season with the salt, the black pepper, the garlic cloves, the olive oil and add enough water to cover all the ingredients.
Turn the heat to medium-low, cover the pan and bring it to a boil.
After that, stir all the ingredients together very well.
Cover the pan again and let it all cook for another 15 minutes.
After this indicated time, add the canned tuna, stir until incorporated.
Add the grated mozzarella cheese, cover the pan and let it all cook until the cheese has fully melted.
Serve.