SHAKE all ingredients with ice and fine strain into pre-prepared glass.
Find a lemon which fits into a small wineglass tightly enough to act as a watertight extension to the glass. Cut off both ends of the fruit and carefully remove the pulp to leave a barrel-shaped shell of skin. Place in the top of the glass. Wet the edge of the glass and exposed fruit shell with sugar syrup and dip in caster sugar to frost the edge of both peel and glass. Leave for a couple of hours to form a hard crust.
Some cocktail historians, Ted Haigh included, consider the Crusta the forerunner of the Sidecar and in turn the Margarita.