Add mint leaves and simple syrup to shaker, muddle gently.
Add gin, elderflower liqueur (or cordial), and lemon juice.
Fill shaker with ice and shake for 10-15 seconds.
Strain into a rocks glass filled with fresh ice.
Garnish with a mint sprig and lemon twist or wedge.
Notes
Fresh mint and lemon juice make a big difference in flavor. Elderflower cordial works well for a non-alcoholic version. Adjust sweetness with more or less simple syrup. A splash of club soda can be added for extra fizz.