Add the mint leaves and lime juice into a cocktail shaker and muddle the mint well to release the oils. Add the remaining ingredients: tequila, triple sec, and rhubarb syrup along with 1/2 cup of ice. Shake well and pour into a glass as is or over ice.
*If you like it sweeter, add another ½ oz of simple syrup.
Add the sugar, water and chopped rhubarb into a small saucepan. Place over medium heat and let cook for 5-10 minutes until sugar is dissolved and rhubarb becomes soft. Shut off heat and add in the mint leaves. Let steep for another 5 minutes.
Strain the mixture and place in the fridge to cool.