Preheat the oven to 200°C/gas 6. Cut the rhubarb into 6cm pieces and place in an ovenproof dish along with the sugar. Add the zest and juice of both oranges, then cover with foil and roast in the oven for 15–20 minutes, or until softened – the cooking time may vary, depending on the thickness of your rhubarb. Taste it and add a little more sugar if you like.
Transfer the rhubarb to a plate, set aside, and pour the roasting juices into a pan. Boil it for a few minutes until it forms a thick syrup, then set this aside too.
To make the custard, pour the milk, cream and 4 tablespoons of the sugar into a pan. Split the vanilla pod in half and scrape out the seeds, adding both to the pan. Bring it to the boil over medium heat, stirring continuously, then remove from the heat and leave to cool slightly.
Place the egg yolks in a large bowl and beat in the remaining sugar to combine. Remove the vanilla pod from the milk, then slowly whisk the warm milk mixture into the yolks. Return the custard to the pan and place over a low heat for a few minutes, stirring continuously, until it coats the back of a spoon. Sieve it into a clean bowl, cover the surface with a layer of clingfilm to stop it forming a skin, and set aside.
Make the meringues by placing the egg whites in a clean bowl and beating them with an electric whisk until they form firm peaks. Continue whisking as you add the sugar, then turn up the mixer and whisk for about 7 minutes, or until they’re white and glossy.
To poach the meringues, pour the milk into a wide, shallow saucepan over a medium heat and warm it through until it just begins to simmer. Float 3 or 4 heaped tablespoons of the meringue mixture on the hot milk, making sure they are well spaced out to allow them to expand. Cook for about 30 seconds, then turn over and cook for 30 seconds more, or until they’re firm.
Using a slotted spoon, transfer the meringues to a plate lined with kitchen paper, to absorb any milk. Repeat with the remaining meringue mixture, discarding the milk once you’ve finished.
Spoon the custard into serving bowls, top with the roasted rhubarb – stirring a little for a rippled effect, if you like – then finish by placing a meringue island on top. Drizzle with rhubarb syrup and serve right away.