Mince the garlic cloves finely and roughly chop the parsley (separating leaves from stems—you'll use the stems for the sauce and save the leaves for garnish). Pit and halve the Kalamata olives if they're whole. Bring a large pot of salted water to a boil for the pasta—this takes time, so start it now while you prepare everything else. The high heat will build flavor in your pasta water.

Heat the olive oil in a large, shallow pan over medium heat. Add the minced garlic, capers, anchovy fillets, parsley stems, and chili flakes all at once. Cook gently for 2 minutes, stirring occasionally—you're not looking for color here, just to infuse the oil with these flavors and let the anchovies begin to dissolve into the oil. I find that adding the anchovies early allows them to melt completely into the sauce, creating a deeply savory base without any fishy aftertaste.

Pour in the diced tomatoes along with 1/2 cup of water, then add the halved olives. Stir well to combine everything, then reduce heat to low-medium and let the sauce simmer gently for 15-20 minutes. The tomatoes will break down slightly, the flavors will meld together, and the sauce will thicken just enough to coat the pasta beautifully. At this point, add the dried oregano and season with salt and pepper to taste.

Once your water is at a rolling boil, add the spaghetti and cook according to package directions until al dente—usually 9-11 minutes depending on the brand. I always taste a strand about a minute before the package time suggests; the pasta should have a slight firmness in the center when bitten. Reserve about 1 cup of pasta cooking water before draining, as this starchy water will help the sauce cling to the pasta.

Transfer the drained pasta directly into the simmering sauce, then add the canned tuna (drained if packed in water). Gently toss everything together for about 30 seconds to 1 minute, adding a splash of reserved pasta water if the sauce seems too thick—you want a light coating that moves with the pasta, not a thick cling. The heat from the sauce will gently warm the tuna without breaking it apart.

Divide the pasta among serving bowls and garnish each portion with the reserved fresh parsley leaves. A final crack of black pepper and a small drizzle of good olive oil adds a luxurious finish if you like.