In a mixing glass or cocktail shaker filled with ice, combine 2½ oz. London dry gin and ½ oz. dry vermouth. Stir until well chilled, at least 30 seconds.
STEP 2
Strain into a chilled martini glass.
STEP 3
Garnish with a green olive or a wide strip of lemon peel. If using lemon, pinch the peel to express its oils, then brush the rim and drop it in.