Cover and shake vigorously, or stir, until combined and chilled, about 30 seconds. (In general, by the time the shaker mists up the drink is ready.)
Strain into a chilled cocktail glass.
Add twist and serve.
Remove the outer peel of the lemon with a vegetable peeler, taking care not to include too much of the bitter white pith. Finely chop the lemon peel and ginger in a food processor.
Transfer the lemon-ginger mixture to a medium saucepan, add the sugar and water and bring to a boil. Reduce heat, and simmer, partially covered, for 15 minutes. Strain the mixture and cool. Cover and refrigerate for up to 1 week.