Preheat the oven to 176°C, line a 13"x 9" sheet pan with baking parchment and lightly grease it.
In a bowl whisk together the flours, salt, baking powder and baking soda.
In another bowl, whisk the sugar with lemon zest and olive oil, beating the eggs in one at a time till you have a creamy, frothy mixture. Add the vanilla.
Mix in the lemon juice and Greek yoghurt and whisk till the yoghurt is incorporated into the egg mixture.
Gently fold in the dry mixture with spatula making sure there are no lumps of flour in the batter. Take to use a light hand just till the batter is a homogenous mixture. Overmixing will yield a dense cake.
Pour the cake into the prepared pan and bake for 30-35 minutes.
While the cake bakes, make the syrup by gently heating the lemon juice with the castor sugar in a pan just till the sugar dissolves. Do not boil.
Cool the cake on a wire rack for 10 minutes and then drizzle the syrup all over the cake evenly. Let it soak and allow the cake to cool fully before slicing.