Bring a large pot of generously salted water to a boil. Add the pasta and cook until just al dente, about 1 to 2 minutes less than the package suggests. Reserve a small cup of pasta water before draining. Drain and set aside.

Heat 3 tablespoons of olive oil in a large oven-safe skillet over medium heat. Add the finely chopped onion and cook for 5 to 7 minutes, stirring occasionally, until soft and translucent.

Add the minced garlic and stir for about 60 seconds until fragrant. Watch carefully to avoid burning.

Pour in the canned or fresh tomatoes, breaking them up with a wooden spoon. Season with salt and black pepper. Simmer uncovered for 15 to 20 minutes, stirring occasionally, until the sauce thickens.

Add the drained pasta to the skillet and toss gently until well coated. If the sauce is too thick, add a splash of reserved pasta water to loosen.

Remove from heat. Stir in the diced fresh mozzarella and freshly torn basil leaves. The residual heat will melt the cheese into creamy pockets. Taste and adjust seasoning.

(Optional bake) Preheat oven to 350 degrees F (180 degrees C). Transfer pasta to a lightly greased baking dish if not already oven-safe. Sprinkle grated Parmesan evenly over the top. Bake for 15 to 20 minutes until golden and bubbling. Rest 5 minutes before serving.