Combine all the ingredients in a small pot. Bring to a boil then reduce to a simmer. Simmer until rhubarb is soft and the mixture has thickened. Remove from the heat and strain through a fine mesh sieve. Pour into a jar or bottle and let cool completely. The syrup will keep covered in the fridge for two weeks. This will make enough for 4 -6 drinks.
Add 5 large mint leaves to a cocktail shaker. Fill half full with ice and muddle with the end of a wooden spoon. Add ¼ cup rhubarb simple syrup. 1 oz. white rum and 3 tablespoons lime juice. Shake until well chilled.
Strain into a glass. Fill ⅔ full with ice cubes and top with sparkling water or ginger ale. Garnish with a mint sprig and/or a slice of lime and a pinch of nutmeg (optional).
Non-Alcoholic Cocktail (Mocktail)
Simply follow all directions, but omit the rum. Add extra sparkling water or ginger ale if desired.