Wash the rhubarb and bring to a boil with the water, sugar and a few sprigs of rosemary. Reduce the heat slightly and simmer for about 15 minutes until the rhubarb pieces fall apart.
Allow to cool slightly and then pour through a fine-mesh sieve, filtering out any suspended matter and leaving only the red rhubarb juice. Add lemon juice and stir. If it's still too sweet, just add a little more lemon juice.
Let the syrup cool down completely.
To serve, fill a glass or carafe 1/3 to 1/2 (the amount depends on your personal taste) with the rhubarb-rosemary syrup and top up with sparkling water.
Garnish with a lime wedge and a sprig of fresh rosemary.