Preheat the oven to 180C/160C fan/Gas 4.
Grease a 11 x 21 cm loaf tin (1.3 L capacity).
Mash the avocados (a blender works well).
Place mashed avocado into a mixing bowl and beat in the butter, sugar, egg, crystallised ginger and vanilla extract. (I used an electric whisk).
Add the bicarbonate of soda and salt and mix in the flour last.
Pour into the prepared tin.
Bake for 55 minutes to 60 minutes until springs back to the touch.
Remove from oven and allow to cool for a couple of minutes before turning onto a wire rack to cool.
Prepare drizzle by adding stirring enough lemon juice into the icing sugar to form a thick glossy icing.
Pour drizzle over the cooled cake and decorate with lemon jelly slices if using.
Allow icing to set, slice and serve.