Lightly bash the ginger, then cut into chunky size pieces. Add lemongrass, ginger and a little palm sugar to water; bring to the boil, then let simmer for around an hour. You may need to turn up the heat to make it thicker and more of a syrup consistency toward the end. Strain and set aside.

Preheat oven to 180C. Grease and line base and sides of a 20cm round cake pan.

Beat butter and caster sugar until pale and fluffy, the beat in eggs, one at a time; it may look a bit curdled, but don't worry. Gently fold in lemon zest and flour, alternating with the yoghurt and lemon juice. Using a spatula, scrape mixture into the prepared pan. Arrange the apple slices on top of the cake, then bake for 45 minutes or until golden brown and a skewer inserted into the centre of the cake comes out clean.

Remove cake from oven, but leave it in the pan. Using a fine skewer, pierce cake all over 30 times. Spoon hot syrup over the cake so it soaks evenly.

Cool, then serve.