Get yourself a small pan and throw in the rhubarb, sugar an da couple of tablespoons of water.
Put a lid on top, bring to the boil and simmer for a couple of minutes. Remove the lid and simmer for a few more minutes, stirring occasionally, until you get a thick compote consistency.
Wiz up with an immersion blender or in a standard blender until you have a lovely smooth puree.
Leave to cool, then stir again and divide the puree between six glasses. Pour over your Prosecco or Champagne, stirring as you pour with a long spoon or something similar, until the glass is three-quarters full.
Top it up with bubbles and your done!