How to Make It
In a jar, combine the vermouth and cocktail onions with their pickling liquid. Refrigerate for 4 days; shake the jar once a day. Pour the infused vermouth through a fine strainer into a jar, pressing down on the onions to extract as much liquid as possible. Reserve the onions for another use.
In a small saucepan, blanch the rosemary in boiling water for 30 seconds. Transfer to ice water to cool. Remove the leaves and transfer to a blender; discard the stems. Add the Simple Syrup and puree until smooth, about 2 minutes. Strain the syrup through cheesecloth into a jar.
In a large pitcher, combine the gin, 10 ounces of the pickled-onion vermouth, 2 ounces of the rosemary syrup and the lemon bitters; refrigerate until chilled, about 2 hours. Fill the pitcher with ice and stir well. Strain into 8 chilled martini glasses and garnish each drink with the skewered cocktail onions.