In a large pot, bring salted water to a gentle boil and add spaghetti. It takes about 8–10 minutes until the strands are al dente and still have a little bite, since they’ll finish cooking in the sauce.
In a large pan, heat two spoonfuls of oil from the tuna can over medium heat until it shimmers. Add anchovies and let them sizzle for a couple of minutes, breaking them up with a spatula as they soften into the oil.
Toss in the garlic and cook for no more than 30 seconds, watching for the edges to turn light golden. This gives a mellow garlic flavour without any harsh notes.
Drain the tuna and add it to the pan, breaking it into chunks as it warms for two minutes. Pour in a ladle of boiling water from the pasta pot and let it simmer for a minute so the sauce binds beautifully.
Pour in the tomato passata, add capers, and sprinkle chilli flakes. Stir gently and let the sauce bubble over medium-low for 5–7 minutes until it thickens slightly and releases a rich aroma.
Use tongs to lift the drained spaghetti into the sauce, then toss until every strand is coated in that tangy mix. You’ll see a glossy finish when it’s ready.
Taste and add salt only if needed, since anchovies and capers add plenty of saltiness. Scatter chopped parsley over the top, then serve right away to enjoy the best texture and flavour of your tuna pasta.