Put the cashews in a large bowl, pour over boiling water and leave to soak for 1 hr. Meanwhile, blitz the ingredients for the base with a pinch of salt in a food processor. Grease a 23cm tart tin with coconut oil, then press the mix into the base and pop in the fridge to set (about 30 mins).
Drain the cashews and tip into the cleaned out food processor. Add all the remaining topping ingredients, reserving a quarter of the lemon zest in damp kitchen paper to serve, then blitz until smooth. Spoon onto the base and put in the fridge to set completely (about 2 hrs). Just before serving, scatter over the reserved lemon zest.