Cut the rhubarb into even sized pieces and place in a single layer in an ovenproof dish or baking tray, sprinkle with 1 tbsp caster sugar and roast at 200C for about 10-15 minutes until soft.
Place the trifle sponges in the base of an ovenproof dish, pour the raspberry coulis or jam evenly over the trifle sponges and then cover them with the rhubarb and the juices that have gathered in the bottom of the roasting dish.
Scatter over the oat crumble (optional).
Make the custard according to the instructions on the packaging and pour over the rhubarb.
Whisk the egg whites until they are stiff, gradually whisk in the sugar until glossy. Pile the meringue on top of the custard and bake in the oven at 160C for 30 minutes, until golden brown.