Prep the tomatoes: Put the tomatoes on a sheet pan covered with either aluminum foil or parchment paper.
Roast the tomatoes: Spread the tomatoes and roast them in the oven for about 45 minutes, until tomatoes are a little charred, bubbly and soft. Make sure to check on them regularly, flipping them over or tossing them around to prevent from sticking to the foil or paper.
Boil water and cook the pasta: Bring a pot of salted water to boil and cook the spaghetti just short of al dente (follow the instructions on the package).
Save some pasta water and drain the noodles: Reserve about a cup of the salted water aside. Drain your spaghetti but do not rinse it.
Cook the aromatics of the sauce: In a medium size pot, add the olive oil, garlic, red chili pepper flakes and onions and cook on medium heat for 4 minutes.
Make the tomato sauce: Add the roasted tomatoes and tomato sauce to the pan and cook for 3 minutes. Turn the heat off and transfer the ingredients to a blender. Add the butter and puree into a smooth sauce.
Thin the sauce if needed: The sauce can be served as is or you can water down the consistency by adding a little pasta water to it and blending again.
Serve: Serve the spaghetti with fresh parsley on top.
Notes
Save the roasted tomato sauce in a storage container and refrigerate for up to 8 days.