Beetroot, Rhubarb and Ginger Soup

Sharp and sweet, rhubarb and beetroot were made for one another, and this soup is a fitting celebration of their union. I had intended to make a beetroot and apple soup (hitherto a favourite) one day, only to find I had no apples, so substituted the rhubarb instead, and have never looked back. (Though if you want to make the apple variant, simply use 2 Granny Smiths in its place. Or, for a Christmassy version, use 200g of cranberries and an extra 100g of beetroot.) The rhubarb undercuts wha