Place the tomatoes (with juices), butter, and onion halves in a medium saucepan. Bring to a gentle simmer over medium heat.

Reduce heat to maintain a slow, steady simmer. Cook uncovered for about 45 minutes, stirring occasionally and mashing any large tomato pieces with the back of a wooden spoon, until fat droplets appear on the surface and the sauce has thickened.

Remove and discard the onion halves. Season the sauce with salt to taste.

Meanwhile, cook the pasta in a large pot of salted boiling water until al dente. Drain and toss with the sauce.

Serve immediately in warm bowls with freshly grated Parmesan and basil leaves if desired.