"This is very much like Krest® Bitter Lemon (found in Africa) or Schweppes® Bitter Lemon (found in Europe). Unfortunately there is nothing comparable found in the USA, but this comes pretty darn close! This is the perfect thirst quencher for a blistering hot day!"
Combine sugar, water, lemon zest, and lavender in a pot and cover; bring to a boil. Cook mixture until sugar is dissolved, 5 to 10 minutes. Remove pot from heat and cool syrup to room temperature, about 15 minutes.
Pour lemon juice into a pitcher, strain syrup into lemon juice, and stir until concentrate is evenly mixed. Refrigerate concentrate until chilled, at least 1 hour.
Fill serving glasses with ice and pour about 2 ounces concentrate into each glass; top with tonic water and stir.