Heat the olive oil in a 10-inch deep-sided skillet over medium-low. Add in the onions and cook until soft and translucent.

Add in the minced garlic and cook for a minute or two. Add a couple pinches of red pepper flakes and stir. Over the pan, crush the San Marzano tomatoes by hand and add in any juices from the can.

Add a half cup of water to the can, swirl it around to loosen any sauce and then pour it into the pan.

Season with a pinch of kosher salt, stir and cook, uncovered for 20-25 minutes or until the sauce has thickened.

Add in the sliced basil, stir and then turn of the heat and remove the pan while preparing the pasta as directed on the package.

Once the pasta is cooked, reserve a half cup of pasta liquid and drain the pasta in a colander. Return the pan to medium-high heat, pour in the half cup of the pasta water to loosen the sauce up a bit.

Add in the drained pasta, the Parmesan cheese and the two tablespoons of butter. Toss the pasta with the sauce until the cheese and butter has melted.

If making chicken sausage, add sliced, cooked sausage at this point.

Stir and divide among bowls, top with more grated Parmesan cheese and enjoy!