Step 1

In a large pot set over medium heat, combine the whole tomatoes and their juices, salt, basil, onion, Italian seasoning, butter, and red pepper flakes. Bring to a simmer, then reduce the heat to medium low. Cook, stirring occasionally until the sauce has thickened and the onion is softened, about 1 hour.

Step 2

Once the sauce has cooked for about 45 minutes, bring a large pot of water to a boil over high heat and salt it. Add the pasta and cook until al dente according to package instructions. Reserve ¼ cup pasta water, then drain.

Step 3

Using a slotted spoon, remove the onion from the sauce and discard. Using an immersion blender, blend the sauce in the saucepan until mostly smooth. (Alternatively, transfer the sauce to a blender, let cool briefly, and blend until smooth.)

Step 4

Add the pasta and reserved pasta water to the sauce and toss to coat.

Step 5

Divide the pasta among six bowls. Garnish with basil, red pepper flakes, and Parmesan, if using.