Bring a large pot of water to the boil. Add a tablespoon of salt and a splash of oil. Cook the pasta according to the packet directions, until al dente. Drain and return to the pan.
Meanwhile, heat the oil in a large frying pan over low-medium heat. Add the onion, garlic and kemon zest and saute until softened. Add lemon juice, tuna, rocket and capers and toss to combine. Season with salt and pepper.
Add tuna mix to the pasta and toss to combine. Serves 4.