01 - In a saucepan, combine the chopped rhubarb, elderflower cordial, water, and sugar. Bring to a boil over medium heat.
02 - Reduce heat and simmer for about 15 minutes, or until the rhubarb is tender.
03 - Remove from heat and let cool slightly. Then, blend the mixture until smooth using an immersion blender or regular blender.
04 - Chill the soup in the refrigerator for at least 2 hours.
05 - Before serving, swirl in the plain yogurt for a creamy finish.
06 - Serve cold, garnished with fresh mint leaves.