Cook the sauce: Reduce the heat to low and let the sauce simmer uncovered for 35 to 40 minutes, stirring occasionally, until the sauce thickens and the butter separates slightly from the tomatoes.

Reserve pasta water: Just before draining the pasta, scoop out and reserve about 1/2 cup of the starchy pasta water.

Remove the onion: Use tongs to remove and discard the onion halves and smashed garlic cloves from the simmering tomato sauce.

Transfer the pasta: Drain the spaghetti and transfer it directly into the saucepan with the tomato butter sauce.

Toss to coat: Toss the pasta well over low heat for 1 minute, adding a splash of the reserved pasta water if the sauce needs to be loosened.

Finish and serve: Remove the pot from the heat. Stir in the chopped fresh basil and grated parmesan cheese, then serve immediately.