Pare two very thin strips of zest from the orange using a sharp knife, or a peeler and then juice the orange. Tuck the rind in amongst the rhubarb, sprinkle over the brown sugar, and drizzle with the orange juice.
Bake for 20 minutes until the rhubarb is tender. Carefully remove the rhubarb from the dish and strain the juices and set both the juice and the rhubarb aside.
Separate the egg yolks from whites. Place the whites together with the salt into the large bowl of a stand mixer and mix on high speed until soft peaks form.
Add yolks and beat, then add the 140g of caster sugar gradually. Beat well.
Sift in the almond meal, custard powder and baking powder and carefully fold through the egg mixture.
Pour the mixture into the prepared tin and bake for 15 to 20 minutes until golden.
Turn out on to a wire cooling rack and remove baking paper. Allow to cool completely.