Preheat the oven 200C/180C Fan/Gas 6.
Halve and stone the plums and put them, hollow side up, in a heatproof dish.
Break up the meringues and sprinkle them over the top, making sure lots fall into the hollows of the fruit.
Place dollops of mascarpone cheese on top of the meringues and fruit.
Sprinkle with the sugar and place the dish in the oven.
Cook for 20 minutes, removing from the oven when the fruit is tender with a crisp crust.
Serve warm.