Preheat the oven to 150°C/300°F/gas 2 and line a 40cm x 25cm baking tray with a sheet of greaseproof paper.
Put your egg whites into a squeaky clean bowl, making sure there are no little pieces of egg shell or yolk in them. Whisk them until the whites form firm peaks. With your whisk still running, gradually add the sugar and then a pinch of salt. If whisking by hand, add the sugar a little at a time, whisking it well before adding more. When it’s all in, whisk briskly, or on a high setting, for about 7 or 8 minutes, until the mixture is white and glossy. To test whether it’s done you can pinch some between your fingers – if it feels completely smooth, it’s ready; if it’s slightly grainy, it needs more whisking.
Dot each corner of the greaseproof paper with a little blob of meringue, then turn the paper over and stick it to the baking tray. Spoon the rest of the meringue over the paper, using the back of a spoon to swirl and spread it out evenly into an A4-size rectangle. Place in the preheated oven and bake for an hour, or until the meringue is crisp on the outside and a little bit soft and sticky on the inside. Set aside and leave to cool.
Meanwhile, pop the rhubarb pieces in a saucepan with the zest and juice of the orange, stem ginger (if using), caster sugar and vanilla pod and seeds. Bring to the boil, then simmer gently for 10 minutes with the lid on, until the rhubarb is cooked. Leave to cool completely.
When your meringue has completely cooled too, transfer it carefully to a nice serving board or platter. Whip the cream with the sifted icing sugar until it forms smooth, soft peaks, then stir in the yoghurt. Sprinkle half the almonds over the top of the meringue then spoon half the whipped cream mixture over the top and drizzle with some of the rhubarb syrup from the pan.
Divide most of the rhubarb pieces evenly over the top. Pile over the rest of the whipped cream mixture and top with the remaining rhubarb. Drizzle with some more of the rhubarb syrup, then sprinkle over the remaining toasted almonds and finish with some grated orange zest. Serve straight away. If you’re making this in advance, get everything ready and assemble at the last minute so the meringue stays nice and crispy. Enjoy!