Gratin with Summer Fruit

Rinse peaches and plums, cut in half, remove seeds and cut into cubes. Rinse blackberries and drain. Mix the fruits together and place in a baking dish. Mix mascarpone with 2 tablespoons sugar. Separate egg and beat the egg white until stiff. Fold into the mascarpone mixture. Beat egg yolk with the lemon zest and the cream. Stir in 2 tablespoons sugar. Divide the mascarpone mixture evenly over the fruit. Pour yolk-cream mixture over. Sprinkle with the remaining sugar. Bake in preheated oven until the sugar starts to brown, about 25 minutes. Serve hot.