Shrubs, or fruit-and-vinegar syrups, are a great way to add acidity and complexity to balance the sweetness of cocktails and alcohol-free beverages. This refreshing libation features tart pickled watermelon shrub, sweet melon liqueur and fizzy club soda.
Bring vinegar and pickling spice to boil in small saucepan for 2 minutes. Strain pickling liquid through cheesecloth-lined strainer into liquid measuring cup. Fold up ends of cheesecloth to form a spice pouch; tie tightly with string.
Place watermelon slices in large resealable plastic bag or jar large enough to hold all of the slices. Pour melon liqueur, gin, pickled vinegar and spice pouch into bag or jar. Seal tightly. Let stand at least 30 minutes to allow flavors to develop.
For each cocktail, pour 1 cup of the pickled melon mixture into tall ice-filled beverage glass. Top each glass with 1/4 cup of the seltzer. Garnish with a pickled watermelon slice. Repeat with remaining ingredients to make 6 cocktails.