1.Place rhubarb, sugar, water, nutmeg, vanilla bean and scraped seeds in a medium saucepan over medium heat, stirring until sugar dissolves. Bring to the boil. Reduce heat, simmer uncovered, for 5 minutes. Cool.
2.Strain rhubarb mixture over a bowl and reserve syrup. Place lime and mint in a large serving jug. Crush with a muddling stick or rolling pin to release the juice. Add reserved rhubarb syrup, rum, elderflower cordial and ice cubes. Top with soda water.
You can reserve the strained rhubarb to flavour smoothies, or spoon on top of porridge, cereal or yoghurt.Note