Heat the oil in a saucepan over medium-low heat. Add the onion and a pinch of salt and fry gently for 7 mins or until softened and turning translucent. Add 350g of the tomatoes, the pasta, and veg stock to the pan and bring to the boil, then reduce to a simmer and cook for 15 mins, uncovered, stirring occasionally. The tomatoes should have broken down and the pasta will be just cooked.
Add the remaining tomatoes and bubble uncovered on medium-high heat for 5 mins or until the liquid has reduced. Gently fold through large flakes of tuna, the mascarpone, parmesan, capers, lemon zest, and parsley as well as salt and a generous grind of black pepper. Place a lid on the pan and leave to sit for 5 mins before serving in deep bowls.