Add ice cubes to a rocks glass and pour the gin and rhubarb syrup over the top.
Squeeze the wedge of lime into the glass and drop it in.
Spank the mint leaves and add them to the glass as well.
Top with tonic water. Stir and serve.
Place the rhubarb, sugar and water into a saucepan and bring to a simmer. Simmer for 20 to 30 minutes or until the rhubarb is completely soft. Strain through a fine strainer, pressing the pulp to get as much liquid out as possible. Store in a glass bottle in the fridge for up to two weeks.