The cashew is the kidney-shaped seed of Anacardium occidentale, a sprawling evergreen tree native to north-eastern Brazil and now grown across the tropics. Sweet, buttery and faintly milky, it is roasted and salted as a snack, blended into rich curries and dairy-free creams, and ground into butter — and it grows in one of the plant world's strangest arrangements, dangling beneath a swollen, edible false fruit.