Moist, fragrant and very green, this cake is perfect with a dollop of loosely whipped cream and maybe a sprinkle of icing sugar.
Preheat the oven to 170C/340F/gas mark 3-4.
Grease and line a 23cm round baking tin. Place the pistachios on a dry baking tray and toast in the oven for 3 minutes until they have a greasy shine and a nutty aroma. Remove and set aside to cool.
Bash the cardamom pods using a pestle and mortar, discard the husks, and grind the seeds to a fine powder.
Tip the pistachios and cardamom powder into a food processor and pulse until they're finely ground. Pour the pistachios into a large bowl with the polenta and bicarbonate of soda and mix until evenly combined.
Place the butter and olive oil into a small pan on a low heat until the butter has melted but not boiled. Leave to cool slightly.
In a second large bowl, whisk the eggs and sugar together until light, fluffy and thick enough to slowly fall off the whisk in a ribbonon the surface of the mixture. Very slowly trickle in the butter and oil, slowly whisking it into the eggs until fully incorporated, then gently fold through the nut and polenta mixture. Zest the lemon, then halve and add the juice and zest to the mix. Stir to combine.
Pour into the greased tin and bake for 45-50 minutes. Check that a skewer comes out clean and the surface springs back a little when you press in the centre. The cake will have risen with a golden top, but don't be upset if the middle has collapsed slightly or even cracked.
Rest in the tin until cool, and serve in thick slices with a dollop of cream. This cake is so moist that it actually improves the flavour to keep for a day or two. However, its vibrant green colour will start to fade.