Go to the trouble of getting all the vinegars for this drinkable brine; you can continue to add onions to it over time, and it just gets better.
- ½ cup apple cider vinegar
- ¼ cup unseasoned rice vinegar
- 1 tablespoon black peppercorns
- 1 tablespoon coriander seeds
- 1 tablespoon juniper berries
- ⅛ teaspoon crushed red pepper flakes
- 1 pound small pearl onions, peeled, trimmed
- 8 ounces gin (preferably Beefeater)
- 2 ounces blanc vermouth (preferably Dolin)
- 2 ounces dry vermouth (preferably Dolin)
Whisk apple cider vinegar, red wine vinegar, rice vinegar, white wine vinegar, sugar, salt, peppercorns, coriander seeds, juniper berries, red pepper flakes, garlic, bay leaf, and 1¼ cups water in a medium bowl until salt and sugar dissolve. Pour brine into an airtight nonreactive container and add onions. Cover and chill at least 1 week. Flavors will deepen the longer the onions sit; the liquid starts getting really delicious after 1 month.
Do Ahead: Pickled onions can be made 3 months ahead. Keep chilled.
For each cocktail, pour 2 oz. gin, ½ oz. blanc vermouth, ½ oz. dry vermouth, and ¼ oz. onion pickling liquid over 4 cups ice in a large measuring or stirring glass and stir 1 minute. (Yes, a full minute. This dilution is key to rounding out the flavors of the finished drink.) Strain into a martini glass and garnish with a pickled onion.