Preheat the oven to 180°C fan-forced (200°C conventional). Grease a rectangular cake tin and line with baking paper.
Whisk the butter and 160 g of sugar in a large bowl until pale and smooth.
Slowly add the eggs, adding a tablespoon of the almond meal just before the last bit of egg – this will stabilise the mix and stop it from splitting when the juice is added – and then mix in the orange zest and juice.
In another bowl, combine the flour, baking powder, salt and remaining almond meal. Fold the dry ingredients into the butter mix and then spoon the mix into the prepared tin. Arrange the rhubarb on top of the cake.
Break the rosemary into 6–7 pieces and spike into the mix. Sprinkle 3 teaspoons of sugar on top of the cake and bake for about 50 minutes.
It’s ready when a skewer comes out clean. Set aside to cool.