Prepare the Pasta: Bring a large pot of salted water to a boil. Add the pasta and cook it according to the package instructions, but reduce the cooking time by one minute to keep it slightly firm. Drain the pasta and rinse under cold water to stop cooking and prevent sticking. Set aside.
Sauté Aromatics: Heat an oven-safe skillet over medium-high heat. Add the olive oil, then sauté the minced garlic and diced onion until fragrant and golden, about 3 minutes. Stir frequently to avoid burning and to bring out the sweetness of the onions.
Build the Tomato Sauce: Pour in the crushed tomatoes, followed by the water and tomato paste (if using). Sprinkle in the dried herbs, salt, and black pepper. Stir well to combine. Bring the mixture to a gentle simmer, then reduce the heat to medium and let it cook vigorously for 5 minutes, stirring occasionally, until the sauce thickens slightly.
Season and Adjust: Taste the sauce and adjust salt as needed. If the sauce tastes too acidic, add a small pinch of sugar to balance the flavors.
Combine Pasta and Sauce: Turn off the heat. Add the cooked pasta directly into the sauce and stir until evenly coated.
Fold in Tuna: Gently fold in the drained chunk tuna, taking care to keep some pieces intact rather than breaking it all up. This creates pleasant bites of tuna throughout the dish.
Assemble and Bake: Drizzle a little olive oil over the top of the mixture, then sprinkle the shredded mozzarella evenly across the surface. Place the skillet (or transfer to a baking dish if preferred) into a preheated oven at 180°C (350°F). Bake for about 20 minutes, until the cheese is melted, bubbly, and golden with some crispy edges.
Rest and Serve: Remove from the oven and let it rest for 3 minutes before serving. This allows the bake to set slightly for easier portioning.