Mix together the mascarpone and pesto. Use a small, sharp knife to make a horizontal slit in the side of each chicken breast to form a pocket. Fill the pockets with the mascarpone mixture.
Season the chicken and rub with 1 tablespoon of the oil then turn in the breadcrumbs until well coated. Place in a baking tray, drizzle over another tablespoon of oil and cook in a preheated oven, 200°C (400°F), Gas Mark 6, for 10 minutes.
Add the tomatoes to the baking sheet, season and drizzle with the remaining oil. Return to the oven for a further 5 minutes or until the chicken is cooked through. Scatter over the pine nuts and serve with crusty bread, if liked.