Place some crushed ice into a wide-rimmed glass, and set aside.
Place 1 cup of crushed ice into a Boston-style cocktail shaker. Pour in the vermouth and swirl it around, making as much contact as possible with the ice. Using a strainer, pour the vermouth out. Add the gin and stir well to combine.
Remove the ice from your serving glass and add the olive. Using the strainer, pour the gin into the serving glass.