Shake, strain, up, mint garnish.
This is Imbibe Magazine's version of a recipe whose earliest source, as far as I know, is the Savoy. The original calls, confusingly, for the juice of either one lemon or one half lime, which would seem to produce two very different drinks. Imbibe goes with half of a modern lime and adds a mint sprig garnish. Erik Ellestad made it with the juice of a whole lemon and found it, against expectations, balanced. More here: http://savoystomp.com/2008/10/06/fallen-angel-cocktail/