Heat oven to 200C/180C fan/ gas 6. Place the tomatoes, cut-side up, in a roasting dish and drizzle with the vinegar, oregano and some seasoning. Roast for 1 hr 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 stock. Simmer for 5 mins, then 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 grill to High. Season the sauce and stir in the basil and pasta. Mix, then tip into an ovenproof dish and cover with the cheese. Grill for a few mins until golden and bubbling.