Get yourself a small pan and throw in the rhubarb, sugar and a 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, or until you get a thick compote consistency, stirring occasionally.
Whiz up with a hand blender or in a liquidizer until you have a lovely smooth purée. Leave to cool, then stir again and divide the purée 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 you’re done. Cheers!
PS I really like serving this cold cold cold on a hot day, so I put the bubbly into the freezer about 40 minutes beforehand. Don’t forget it’s there!