Heat 4 tablespoons olive oil in a large skillet over low heat.
Add 2 garlic cloves, 150 g pitted black olives, and 2 teaspoons rinsed capers to the skillet and sauté gently for about 2 minutes; add 1/2 teaspoon red chili flakes if using.
Pour in 1 (420 g) can whole San Marzano tomatoes and add 2 basil leaves; use the back of a wooden spoon to break the tomatoes into chunks.
Cover and simmer the sauce on low for about 15 minutes.
In the last few minutes of cooking, remove and discard the garlic cloves, then add 280 g drained canned tuna and gently stir to combine; season with sea salt and freshly cracked black pepper to taste.
Meanwhile, bring a large pot of lightly salted water to a boil and cook 380 g spaghetti according to package directions.
Drain the pasta about 2 minutes before the package time for al dente, reserving a little pasta water if desired.
Toss the drained spaghetti into the tuna puttanesca sauce, mixing until evenly coated; add a splash of reserved pasta water if needed to loosen the sauce.
Serve topped with 2 tablespoons chopped fresh parsley and additional freshly cracked black pepper if desired.