Add all ingredients into a shaker with ice and shake.
Strain into a rocks glass over crushed ice.
Garnish with a mint sprig.
*Rhubarb syrup: Add 4 cups roughly chopped rhubarb, 8 cups sugar and 2 qt (64 oz) water into a saucepan and cook over low heat, stirring until sugar dissolves. Cook for 5 minutes more, turn off heat and let cool completely. Strain and store in refrigerator up to a month.