Make the Keto Simple Syrup and add the diced rhubarb to it and allow it to simmer on medium low until the rhubarb breaks up. Remove from the heat and allow it to steep for an hour and cool in the fridge after straining until ready to use. Keep for 7 days.
For the cocktail, in a Collins glass muddle the 4-5 mint leaves in the bottom of the glass. Add in the rhubarb syrup, lime juice, and rum. Add the ice and top with the club soda. Garnish with extra mint stalks and rhubarb.