1. To start, let's make the rhubarb syrup. Peel and chop 500g (1.1lb) of fresh rhubarb, making sure to wash it thoroughly beforehand.

2. Combine the chopped rhubarb with 250g (1.1 cups) granulated sugar and 500ml (2.1 cups) water in a large saucepan.

3. Bring the mixture to a boil over high heat, then reduce the heat to medium-low and simmer for 15 minutes, allowing the flavors to meld together.

4. Next, strain the syrup through a fine-mesh sieve, pressing on the solids to extract as much liquid as possible. Discard the solids.

5. Let the syrup cool to room temperature. This step is crucial, as you want the syrup to be completely cooled before moving on to the next step.

6. Pour the cooled syrup into a large pitcher or individual glasses filled with ice cubes.

7. Top the syrup with chilled Prosecco, about 125ml (4.2oz) per serving. You can adjust the amount to your taste, but this should give you a good starting point.

8. Add a few sprigs of fresh lemon balm to each glass for garnish.

9. Serve immediately and enjoy! If you're feeling fancy, consider serving the drink with toasted bread and crayfish tails on the side.