Heat the olive oil in a large skillet over medium heat, add the onion and sauté for about 10 minutes, stirring occasionally, until the onions become soft.
Add the anchovies to the pan and stir until they break down, about 30 seconds or so.
Add the garlic and the tomato paste at the same time, and give everything a big stir to combine the mixture. Cook for 1-2 minutes, until the whole mixture starts to caramelize. Season generously with salt.
Add your canned tomatoes and their juices to the pan, stir everything around to deglaze the pan, and add another generous pinch or two of salt, and simmer at a low heat for 15-20 minutes, until the sauce has slightly thickened.
Season to taste with additional salt, freshly cracked black pepper, and a big pinch of crushed red chili flakes for spice if desired. Throw in a knob of butter at the end and toss to combine if desired.