To make the rhubarb syrup mix together the water, sugar and rhubarb in a medium pot and place on medium heat until it comes up to a boil. Reduce heat to a simmer and cook until the rhubarb has broken apart, about 15-20 min. Set aside to cool and then drain the rhubarb through a fine mesh strainer and reserve the syrup.

On a small plate squeeze some fresh lime out and turn two glasses upside to moisten the rim. On a separate plate add the salt and rim the glasses with the salt. Fill the glasses with ice. Mix the cocktail ingredients into a shaker on ice. Shake and then pour into prepared glasses.