Heat the oven to 400°F Place the tomatoes, cut side up, in a roasting dish and drizzle with the vinegar, oregano and some salt and pepper. Roast for 1 hour until soft.
Heat the oil in a saucepan, then cook the onion for 5 mins until soft. Add the garlic and cook for 1 min more. When the tomatoes are ready, tip into the pan with the broth. Simmer for 5 mins, the add the mascarpone. Remove from the heat and blitz with a hand blender until smooth.
Meanwhile cook the pasta following pack instructions, drain and set aside in a colander. Heat the broiler to high. Season the sauce and stir in the basil and pasta. Mix and then tip into an oven proof dish and cover with the cover with the cheese. Broil for a few mins until golden and bubbling. Serve.