Heat the pasta water: Bring a large pot of salted water to a boil (1 tablespoon of salt for every 2 quarts of water). While the water is heating, start making the sauce.

Cook onions, anchovies, garlic: Heat the olive oil over medium heat in a large, deep sauté pan. When the oil is hot, cook the onions until they're soft and translucent, about 4-5 minutes. While the onions are cooking, stir in the chopped anchovies along with some of the oil from the can. Add the finely chopped garlic and cook another minute.

Make the sauce: Mix in the tomato paste and cook it for 2 minutes, stirring occasionally.Add the crushed tomatoes, oregano, chili pepper flakes, olives, and capers. Bring the sauce to a simmer, then lower the heat to low to maintain a gentle simmer, 10 to 15 minutes.

Cook the spaghetti: When the salted pasta water is at a rolling boil, add the pasta. Cook the pasta according to the package instructions, to al dente, cooked but still slightly firm. Before draining, save about ½ cup of the pasta cooking water to add back later, if needed.

Finish the sauce: Stir the parsley into the pasta sauce. Add some pasta water into the sauce to thin it if it has become too thick.

Serve: Drain the pasta and put in a large bowl. If you want, mix a little olive oil into the pasta so the pasta doesn't stick together. Add a ladle's worth of sauce to the pasta and mix to combine. Serve in shallow bowls with more sauce on top.