Pit the olives if necessary and then coarsely chop both the anchovies and olives.
Heat a 10-inch/26-cm frying pan over medium heat. Once heated, add two tablespoons of the olive oil to the pan. Once the oil is heated (about 30 seconds), add the onions. Sauté the onions until they're translucent.
Add the garlic to the pan and cook until just fragrant (about a minute). Add a small pinch of salt and pepper (to taste) to the garlic-onion mixture.
Add the anchovies, olives and capers to the pan and sauté the mixture together for about two minutes. Optionally add the pepper flakes.
Add the tomato puree and about a cup of water to the pan. Turn the heat to low and cook for about an hour. Add additional water if the mixture gets too thick.
After an hour, keep cooking until the sauce gets thick. Once the sauce arrives at your desired thickness, cover the sauce to keep it warm or keep it over a small fire if it gets cool.
About 5 minutes before the sauce is done, heat the skillet over a medium fire for about five minutes or until hot.
Add the last tablespoon of olive oil and heat it for about 30 seconds. Add the swordfish steaks to the skillets, pressing them gently with a spatula so that they're flat against the bottom of the pan.
Cook the steaks until they're done and brown on each side (about four minutes per side). Use a cake tester to make sure the steaks are done. (The end of the tester should feel hot to the touch after inserting.) Immediately place the finished steaks on a plate to rest (about one minute).
Plate the dish by delicately spooning a pool of sauce on a flat round plate. Gently lay the cooked swordfish over the sauce and garnish with clipped or torn basil.
Notes
You can use another meaty fish like monkfish or mahi-mahi instead of swordfish.
You can use good quality canned pureed tomatoes or canned whole tomatoes that you crush yourself instead of passata.