Remove the leaves from a sprig of rosemary and muddle, or lightly crush them at the bottom of a cocktail shaker. Don't press too hard, rosemary is a very pungent herb, and you don't want it to overpower the drink. Add the rum, simple syrup and grapefruit juice. Add ice to fill the shaker and shake well, then strain over fresh ice in a tall glass. Garnish with a lime wedge and a fresh sprig of rosemary.

To make the simple syrup, put the rhubarb, water and sugar in a sauce pan. Bring to a boil, stirring to dissolve the sugar.

Boil gently for about 10 minutes.

Strain out the rhubarb, and put the syrup back on the stove to reduce a bit further. Boil gently for about 5 to 8 minutes, until the syrup is reduced to about 1 1/4 to 1 1/2 cups.

Let cool and then put in a container with a lid and refrigerate until ready to use.