Preheat oven to 375 degrees F.

Cut tomatoes in half and place on a baking sheet.

Drizzle tomatoes with olive oil and sprinkle with salt.

Place a garlic slice on each tomato half.

Bake tomatoes for one hour, until shriveled but still juicy.

Remove from oven and set aside.

Place a large pot of water on high heat for your pasta.

Once the water comes to a boil, add your pasta and cook according to package instructions, reserving about 3/4 cup of pasta water before draining.

In a large, heavy-bottomed skillet over medium heat, add about 2 tablespoons of olive oil.

Add the remaining garlic and cook until fragrant.

Add the chopped yellow onion and basil, cooking until the garlic and onions are translucent and the oil has been infused.

Add the roasted tomatoes and any collected juices, as well as the red pepper flakes to the onion pan.

Stir to disintegrate the tomatoes and create a smooth mixture.

Add about 3/4 cup of pasta water to the tomato sauce.

Drain the pasta.

Stir the butter into your sauce mixture until it melts.

Toss the pasta into the skillet to coat.

Serve the pasta covered in sauce with shaved Parmigiano Reggiano and more chopped basil.