Using a hand whisk or electric beaters, whisk the sugar and eggs together until well combined and slightly thickened. Add the oil and whisk again. Stir in the lemon zest.
Sieve in the dry ingredients and fold in.
Add the kefir and stir gently until just combined.
Strain a splash of the spinach puree through a sieve and put aside. Reunite any pulp remaining in the sieve with the rest, then gently stir into the cake batter until everything is nicely green.
Pour the batter into two 20cm / 8" round silicon moulds or lined cake tins and bake in the centre of the oven for about 40 minutes or when an inserted skewer comes out clean.
Remove from the oven and leave to cool for 10 minutes or so. Turn out onto a wire rack and leave to cool completely.
Lemon Ricotta Filling
Beat all the ingredients together until combined and smooth.
Spread over one of the cooled sponge halves and place the other on top.
Lemon Spinach Icing
Sieve the icing sugar into a bowl.
Add the lemon juice and a little of the spinach juice that was set aside from the puree. Mix until the icing is thin enough to spread over the cake, but not so much that it will pour off.
Spread over the top of the cake, then scatter over the pistachio pieces.
Oven temperatures vary, so your cake may need a bit more or a bit less time.Will keep for a couple of days if you're kitchen isn't too hot.Please note: calories and other nutritional information are per serving. They’re approximate and will depend on serving size and exact ingredients used.