This is a slight adaptation of a really good recipe from the Jamie Oliver 20 minute meals iPhone app...
Pick the basil leaves and place to one side. Finely chop the stalks. Add a splash of olive oil to a large pan over a medium heat. Add the onion, chilli and basil stalks and fry for around 10 minutes until the onions have softened. Meanwhile, cook the pasta in boiling salted water.
Pour the oil from the jar of tuna away, then add the tuna and tinned tomatoes to the pan. Turn up the heat and stir with a wooden spoon. Simmer for a few minutes until the sauce has thickened, then season well with salt and pepper.
Drain the pasta, reserving a little of the water. Tip the drained pasta into the pan of sauce and toss together. If the sauce is a bit thick, add a little of the reserved cooking water to thin it out slightly.
Grate in some lemon zest and squeeze in a little lemon juice. Add the basil leaves and finely grate in half the parmesan. Serve with the rest of the parmesan sprinkled on top.