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Boil the pasta. Meanwhile, tip the tuna and its oil into a large bowl with the pesto. Mash together with a wooden spoon. Stir in a third of the cheese and all of the tomatoes. Heat the grill to high.
When the pasta is cooked, drain and toss through the pesto mix. Tip into a shallow baking dish and scatter with the remaining cheese. Place the dish under the grill for 3-4 mins until just melted, then serve with a green salad and garlic bread.