Place the chicken breasts in an ovenproof baking dish. Sprinkle the stock cubes over the chicken and add some fresh thyme. Add enough cold water to come up to half the height of the chicken. Cover with tin foil.
Bake in the preheated oven for 40 minutes until the chicken is no longer pink in the centre.
Meanwhile, while the chicken is baking, warm a little oil in a frying pan over a medium heat and add the mushrooms. Cook and stir for a few minutes then add the tomatoes. Reduce the temperature to low, cover and allow to sweat. Check from time to time that the tomatoes are cooking down.
When the tomatoes have turned almost to sauce, add the fresh thyme and cook for another 5 minutes. (If there is a lot of liquid from the tomatoes at this stage, remove the cover, increase the heat and stir until the liquid evaporates.) Turn down the heat to low and add the mascarpone cheese. Season to taste.
Remove the chicken from the oven and transfer to a serving plate. Pour the sauce over and serve with crusty bread.