First, put a large pan of salted water on to boil. Cook the pasta in the pan of salted water according to packet instructions. Drain - retaining a cup of the starchy pasta water - and leave to one side.
While it is cooking, chop the onion and add to a large saucepan with 2 of the tablespoons of olive. Cook on a medium heat for one minute.
Chop the tomatoes into small bite-sized pieces and add to the pan. Cook down for 2-3 minutes, then add the olives and tuna. Leave to bubble away for 2-3 more minutes.
Chop the parsley and add to the tuna sauce, then tip in the pasta, retained pasta water and remaining olive oil. Give it a good mix together - and you're done!