Bring a large pan of water to the boil. Cook the pasta according to the pack instructions.
Meanwhile, finely chop the red onion and garlic. Cut the tomatoes into quarters and slice the chicken thinly. Heat the oil in the pan, add the onion, garlic and chicken and cook gently for 5 mins.
Add the tomato paste, chopped tomatoes, olives (if using), a cup of water and the parsley or basil and cook for another 10 mins. Using a wooden spoon, mash the olives slightly as you stir the sauce. Add the pasta and mix everything together.