Heat the oven to 190C/fan 170C/gas 5. Fry the onion and garlic in 3 tbsp oil until softened but not coloured. Add the tomatoes, season, then simmer for 10-15 minutes or 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 pan and fry the chicken on both sides until golden. Transfer to a baking dish and pour over the sauce. Cook for 25-30 minutes or until the chicken is cooked through. Scatter over the rest of the basil.