Heat the oven to 190C/fan 170C/gas 5. Fry the onion and garlic in 3 tbsp of olive oil until softened but not coloured. Add the tomatoes, season then simmer for about 10-15 minutes until thick and glossy. Take off the heat and stir in the mascarpone and half the basil, roughly torn. Heat a little oil in a frying pan then fry the chicken breasts on both sides until golden. Put the chicken in an ovenproof baking dish and pour over the sauce. Cook for 25-30 minutes or until the chicken is cooked through. Scatter over the remaining basil and serve.