Stir 6 ounces orange juice , lemon zest, lime zest, 1 ounce lemon juice , 1 ounce lime juice , 2 ounces demerara syrup, and 4 ounces Pimm's liqueur together in a large glass measuring cup. Cover with plastic and refrigerate at least 2 hours, or up to 24 hours. Strain mixture through a fine mesh sieve into two glasses filled with ice. Top with club soda and garnish with extra slices of lime or lemon, if desired.
Method 2 - Make Right Now
Fill a cocktail shaker halfway with ice. Add 6 ounces orange juice , 1 ounce lemon juice , 1 ounce lime juice , 2 ounces demerara syrup, and 4 ounces Pimm's liqueur (omit lemon and lime zest) and shake well. Pour into two glasses filled with ice. Top with club soda and garnish with extra slices of lime or lemon, if desired.
Method 3 - Large Batch Make-Ahead
Pour 3 cups orange juice, 1/2 cup lemon juice, 1/2 cup lime juice, 1 cup demerara syrup, and 2 cups Pimm's liquer to a large pitcher or bottle (the recipe makes almost 2 quarts/liters, so choose your container accordingly). Refrigerate at least 2 hours until chilled. Pour into glasses filled with ice leaving about 1/4 of the glass empty. Top with club soda and garnish with extra slices of lime or lemon, if desired. Note that this recipe makes approximately 8 drinks.
Notes
To make demerara syrup: Add 1/2 cup water and 1/2 cup demerara sugar (raw cane sugar) to a small saucepan over medium high heat. Stir while bringing to a boil until sugar is dissolved. Remove from heat. Chill before using. Store extra syrup in the refrigerator.