Fill a mixing glass with ice cubes. Add all ingredients. Stir and strain into a chilled cocktail glass. Garnish with a cocktail onion.
The exact origin of the Gibson is unclear, one story postulates a savvy investment banker named Gibson, who would take his clients out for the proverbial three-martini business lunches. He purportedly had the bartender serve him cold water, permitting him to remain sober while his clients became intoxicated; the cocktail onion garnish served to distinguish his beverage from those of his clients.