Preheat the oven to 200°C.
Prepare the compote by placing the raspberries and sugar in a large saucepan. Heat over a moderate heat with 50ml of water and cook until soft; 4-5 minutes. Remove from the heat and press the raspberries through a sieve into a clean saucepan.
Add the rhubarb to that saucepan and heat over a medium heat. Cover with a lid and cook, stirring occasionally, for 8-10 minutes until the rhubarb is soft. Remove from the heat and stir a few times to produce a homogeneous mixture.
Prepare the meringue by whisking the egg whites to soft peaks in a large, clean mixing bowl. Add of the caster sugar, a tablespoon at a time, beating well between additions. Whisk in the cornflour, then the remaining sugar the same as before, whisking well between additions until you have a thick, glossy meringue.
Divide the compote between 4 heatproof serving glasses. Spoon the meringue on top and bake immediately for 10-15 minutes until the top is lightly browned. Remove and allow to cool for a few minutes before serving.