Heat the olive oil in a medium-size saucepan. Add the onion and sauté until it is translucent. Add the garlic and sauté for about 1 minute, or until it is light gold and fragrant. Add the tomatoes, salt, pepper, and red pepper flakes and simmer for about 30 minutes on low heat. Stir often. Put the tuna in a small bowl and break it up well. Add it to the sauce. If you use salt-cured capers, wash them well in cold water and add them to the sauce, otherwise use capers packed in vinegar, drained. Add the basil, chopped olives, and anchovy fillets. Let the sauce simmer for about 5 minutes.

Meanwhile, cook the pasta in lightly salted water until al dente, about 6 minutes; for fettuccine, 1 minute should be enough. Drain the pasta but not too well, leaving in some of the cooking water. Add about 1 cup of sauce and toss. Serve the pasta on hot plates and put more sauce on top.