SHAKE all ingredients with ice and strain back into shaker. DRY SHAKE (without ice) and fine strain into ice-filled glass.
As with most sours, adding a couple of dashes aromatic bitters greatly contributes to the cocktail's flavour but this also adds trace amounts of sugar.
Adapted from the classic serve by yours truly (Simon Difford) in January 2019.
Also see Sours - history, recipes and glassware.