Heat the olive oil in a large skillet over low heat.
Add the garlic cloves, pitted black olives, and rinsed capers; cook gently for 2 minutes until fragrant, stirring occasionally. Add red chili flakes if using.
Pour in the whole canned San Marzano tomatoes and add the basil leaves. Break the tomatoes into chunks with the back of a wooden spoon.
Cover and simmer the sauce over low heat for about 15 minutes, stirring occasionally.
In the last few minutes of cooking, remove and discard the garlic cloves, then add the drained 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. Cook the spaghetti according to package directions, then drain 2 minutes before the suggested time for al dente.
Add the drained spaghetti to the skillet with the tuna puttanesca sauce and toss until well combined.
Serve topped with chopped parsley and extra freshly cracked black pepper if desired.
