What is the only 'nut' that is actually a seed growing on the outside of a fleshy, pear-shaped fruit?

Cashew

The edible cashew kernel develops inside a hard-shelled true fruit attached beneath the pear-shaped cashew apple, which is actually a swollen, fleshy receptacle. Because this true fruit hangs below the cashew apple rather than being enclosed within it, the familiar “nut” appears to dangle from the bottom.