Place the chopped rhubarb in a large non-reactive container (glass, ceramic, or food-grade plastic). Pour over the water and add the sugar and lemon juice. Stir well until the sugar dissolves.
If using baker's yeast, sprinkle it over the top and give it a gentle stir. If using sourdough mother, mix it in thoroughly. Cover with a clean cloth and secure with a rubber band to keep insects out while allowing gases to escape.
Leave at room temperature for 3-5 days, stirring once daily. The baker's yeast version will ferment more quickly and predictably, while the sourdough version will take a bit longer but develop more interesting, complex flavours with subtle sour notes.
When it tastes pleasantly tart and lightly fizzy, strain through muslin or a fine sieve, squeezing the rhubarb to extract all the liquid. Bottle in clean glass bottles with swing-top lids or crown caps, leaving about 5cm headspace.
Leave the bottles at room temperature for 2-3 days to develop carbonation, then refrigerate. This slows fermentation and creates that lovely champagne fizz. Always open carefully over a sink as it can be quite lively!
Best consumed within 2-3 weeks while fresh and fizzy.
