Jerusalem artichokes are weird little roots (from a type of sunflower!) with a texture like potato and a flavour of mushroomy globe artichoke, that matches sublimely with black truffle.
They are great simply roasted, but also liquidise into a silky smooth purée and naturally make this rather wonderful soup. In fact, if you want to make it vegan, you could omit the cream and the texture would still be amazing.
Be warned, it’s nickname (fartichoke) is not without reason, so go easy. You can apparently reduce the, er, consequences but I haven’t tested out this method.