Bring a pot of water to a boil and add 1 tablespoon salt. Add 250 g spaghetti and cook according to the package instructions. Before draining, reserve some pasta cooking water, then drain the spaghetti.
While the pasta cooks, heat 3 tablespoons extra-virgin olive oil in a saucepan. Add ¼ teaspoon dried chili flakes and 2 garlic cloves, halved crosswise. Cook for 1 minute.
Add ½ medium red onion, finely chopped, and 6 cherry tomatoes, halved crosswise. Cook for 3 minutes. Stir in 100 g tomato passata, ¼ teaspoon salt, and cook for 2 minutes.
Add 1 tablespoon capers and 2 tablespoons whole pitted green olives. Cook for 1 minute. Add the drained spaghetti, using a little reserved pasta cooking water if needed to loosen the sauce. Toss and cook for 2 minutes.
Add drained 280 g canned tuna chunks in oil and 1 tablespoon fresh parsley, finely chopped. Gently toss the tuna tomato pasta, and cook for 2 minutes.
Notes
Drained tuna weight: Measure 9 oz / 280 g after draining so the tuna-to-pasta ratio remains consistent.Removable garlic: Keep the garlic in large crosswise halves so every piece is easy to remove before serving.Pasta water: Add it gradually, using only enough to loosen and coat the spaghetti.Tuna texture: Toss gently after adding the tuna so the chunks retain more of their shape.