5 rhubarb stalks, halved length-ways, cut into 5cm pieces
3cm piece ginger, finely grated
1 1/2 cups (330g) caster sugar
4 eggs, separated, plus 2 extra egg whites
1/4 cup (60ml) olive oil
1 tsp vanilla extract
1 cup (150g) plain flour, sifted
1/2 cup (60g) almond meal
1 tsp baking powder
1/4 tsp cream of tartar
Preheat the oven to 160°C. Grease a 22cm round spring-form cake pan and line the base with baking paper. Cover the outside of the pan with foil to prevent any syrup leaking out during cooking. Arrange the rhubarb in the base of the pan, ensuring there are no gaps.
Place ginger, 3/4 cup (165g) sugar and 1/3 cup (80ml) water in a saucepan over medium-high heat, stirring until sugar dissolves. Bring to the boil, then cook for 10 minutes or until thickened but not golden. Pour over the rhubarb and set aside to cool slightly.
Beat egg yolks and 1/2 cup (110g) sugar with electric beaters until thick and pale. Add the oil, milk and vanilla, then beat until just combined. Add the flour, almond meal and baking powder, then beat until well combined. In a clean bowl, whisk the 6 egg whites to soft peaks. Gradually add the cream of tartar and remaining 1/4 cup (55g) sugar, then whisk to stiff peaks. Fold one-third of the meringue into the batter to loosen, then fold in the remainder.
Pour batter over the rhubarb, then bake for 50 minutes or until a skewer inserted into the center comes out clean. Cool cake completely in pan (it will shrink slightly). Invert onto a plate, then serve.