The Southside is a gorgeous pre-Prohibition cocktail that gained many followers among the well-heeled denizens of various East Coast social clubs. It was very likely meant to be a Julep riff, but it’s got enough style to stand on its own.
Over time, Southside recipes shifted from lemon juice to lime juice. Either citrus is lovely, so choose whichever one you prefer.
Muddle the mint and simple syrup in a shaker. Add the remaining ingredients and ice and shake well. Double-strain through a fine mesh strainer into a cocktail or coupe glass.