To prepare the sauce, bring cream and thyme sprig to a boil over medium heat. Lower the heat and let simmer until the cream has reduced by one-third, about 10 minutes. Remove from the heat let sit for 10 minutes. Remove the thyme sprig and throw it away. Heat the cream again, this time adding chicken stock and simmer for five minutes. Add tomato purée and salt and pepper to taste. Take off the heat, stir in thyme leaves. Set aside and keep warm.