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Heat the oil in a saucepan over a 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 a 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.