Put 150g of the pistachios and the sugar in a food processor and whizz until the nuts are very finely ground.
Heat oven to 180C/160C fan/gas 4, grease a 20 x 30cm cake tin and line the base with baking parchment. Tip the pistachio sugar into a big mixing bowl with the butter, flour, baking powder and bicarb, eggs, yogurt and the zest and juice from 2 of the lemons. Beat with an electric whisk until smooth and combined.
Stir in the courgette until everything is well mixed, then scrape into the prepared tin. Bake for 35-40 mins until the cake is risen and golden, and a skewer poked into the centre comes out clean. Leave to cool.
When the cake has fully cooled, remove from the tin and peel off the baking parchment. Sift the icing sugar into a bowl, and gradually stir in enough lemon juice to get a runny-ish consistency. If you run out of juice, carry on mixing with water. Drizzle thickly over the cake, and repeat with lemon curd, if you like. Roughly chop the remaining pistachios and scatter over the icing, along with the remaining lemon zest. Leave to set for 10 mins, then cut into squares and serving with Greek yogurt. Will keep for 3 days in an airtight tin.