Heat oven to 160C/140C fan/gas 3. Break the cake into fine crumbs or place in a food processor and blitz until it resembles breadcrumbs. Place the crumbs in the bottom of a 22cm round ovenproof baking dish. In a jug, whisk together the egg yolks, milk, nutmeg and 50g of the sugar. Pour the custard mixture over the crumbs and leave to soak for 10 mins. Place the dish in the oven and bake for 45 mins, until the custard is set but still has a little wobble.
Meanwhile, make the filling. Place the rhubarb in a saucepan with 100g of the remaining sugar, stem ginger, syrup and 2 tbsp water. Set over a medium heat and cook for 15-20 mins until the rhubarb is tender. Leave to cool. Can be done up to this point the day before.
Heat oven to 200C/180C fan/gas 6. Place the egg whites in a large clean bowl and, using an electric whisk, whisk the whites until soft peaks form. At this stage, begin to slowly add the remaining 100g sugar, beating well between each addition, until all the sugar is incorporated and the meringue is stiff and glossy.
Drain the rhubarb, keeping the syrup, then spoon onto the baked custard. Spoon or pipe the meringue on top and bake for 15-20 mins until the meringue is browned. Serve hot with the rhubarb syrup.