Bring a large pot of heavily salted water to a boil over high heat. Once it boils, add the pasta and cook for 2 minutes less than the package directions so it is still al dente (reserve 1 cup (235 ml) of the pasta cooking water).
Meanwhile, heat 1/4 cup (60 ml) olive oil in a large frying pan over low heat. Add the garlic, stir to coat in oil, and cook until golden, about 3 minutes.
Remove from heat and carefully stir in the olives, tomatoes, a pinch of kosher salt, and the red pepper flakes, if using (the mixture may splatter, so keep an eye out). (Note: You could use a 15-ounce can of canned tomatoes in place of the fresh tomatoes, but you will need to cook the sauce for closer to 10 minutes.)
Return the pan to the stove over medium-low heat. Stir in ⅓ cup (80ml) of the pasta cooking water, smash about half of the tomatoes with the back of a wooden spoon, and cook for about 5 minutes, until the tomatoes collapse and the sauce is loose.
When the pasta is ready, use tongs to add it directly from the frying pan into the sauce (it’s okay if the noodles still have some water on them). Add 1 TBSP of lemon juice, all the zest, and the remaining 2TBSP of olive oil, then toss to coat with tongs. Add more water if needed to make it loose enough to coat the pasta well.
Remove from the heat, add the flaked tuna and all the parsley, and toss to just incorporate. Taste and add salt or pepper as desired. Serve immediately.