Preheat the oven to 150°C.
Grease a 20cm baking dish with butter and dust with caster sugar.
Toast the breadcrumbs in a frying pan until golden-brown.
Mix the toasted breadcrumbs with brown sugar until well combined.
Heat milk, golden caster sugar, lemon zest, and butter in a saucepan until butter melts.
Stir in the breadcrumb mixture and leave to cool.
Beat in the egg yolks and pour into the prepared baking dish.
Bake for 25 to 30 minutes until the top is golden-brown.
Increase oven temperature to 200°C.
Simmer rhubarb with 200g of sugar in a saucepan, then cool.
Spoon the rhubarb over the baked custard base.
Mix jam with 15ml warm water and spoon over the rhubarb layer.
Whisk egg whites to soft peaks, then gradually whisk in 45ml sugar to reach stiff peaks.
Spoon meringue over the pudding and bake for 8 to 10 minutes until golden.
Serve the pudding with lightly-whipped cream.