1. Cook the Pasta: Begin by boiling a large pot of salted water. Once boiling, add the pasta and cook according to the package instructions until al dente. Drain and set aside.
2. Prepare the Sauce: In a large skillet, heat olive oil over medium heat. Add the chopped onions and sauté until translucent.
3. Add Garlic: Stir in the minced garlic and cook for about 1 minute, taking care not to let it burn.
4. Incorporate Tomatoes: Pour in the canned or fresh tomatoes. Stir well and season with salt and pepper. Cook for about 15-20 minutes until the sauce thickens.
5. Mix in Pasta: Add the cooked pasta to the skillet, combining it gently with the sauce until every piece is coated.
6. Add Cheese: Stir in the diced mozzarella, allowing it to melt slightly into the pasta mixture.
7. Garnish: Add freshly torn basil leaves and stir to combine. Taste and adjust seasoning if necessary.
8. Bake (Optional): For an extra cheesy top, transfer the pasta to a baking dish, sprinkle with grated Parmesan, and bake in a preheated oven at 180°C (350°F) for 15-20 minutes, until bubbly and golden.
9. Rest: Remove from the oven and let it rest for a few minutes before serving.
10. Serve: Dish out the Pasta alla Sorrentina, garnishing with additional basil and a drizzle of olive oil if desired.