Pre-heat the oven to 180°C (fan)/200°C/gas mark 6. Grease and line a 900g loaf tin with non stick baking paper. If you leave some of the baking paper hanging over the ends it will be easier to remove your cake.
Grate 2 small-medium courgettes into a colander (you need around 350g grated courgette) and use your hands to squeeze out as much excess water as possible. Leave in the colander and set aside.
Mash a ripe banana into a large mixing bowl. Beat in 2 eggs or flax eggs (see note*), 75g caster sugar and 1 tsp vanilla extract. Add 100ml sunflower oil and beat again until well mixed.
Sift in 250g plain flour, 1 tsp baking powder and 1/2 tsp bicarbonate of soda and fold into the mixture. Fold in 50g ground almonds.
Stir through the grated courgette and lemon zest.
Pour into the prepared loaf tin and bake for 45 minutes until firm to the touch and a skewer inserted in the centre comes out clean. Leave in the tin to cool completely.
Once the cake is completely cool mix together 175g sifted icing sugar with 2 tbsp lemon juice. Pour over the cake and decorate with grated lemon zest if desired. Leave to set.
Store in an airtight container in a cool place.
*To make two flax eggs mix 2 tbsp ground flax seed (linseed) with 5 tbsp cold water. Set aside until you have a gloopy mixture.