Into a medium saucepan add rhubarb, water, sugar, and rosemary. Bring to a boil. Reduce heat to a simmer and cook for about 15 minutes or until rhubarb pieces break down.

Remove from heat and strain with a mesh strainer. Add lemon juice. Chill until ready to serve.

When ready to serve, fill half of the glass with the rhubarb syrup, and top with Perrier or club soda.

Add ice or alcohol if desired. I'd recommend either vodka or white rum.