1 1/2 cups white sugar
1 1/2 cups water
1 lemon, zested
2 sprigs fresh lavender
3 cups lemon juice
4 cups ice cubes, or as needed
1 (11 fluid ounce) bottle cold tonic water, or more as needed
1. 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.
2. 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.
3. Fill serving glasses with ice and pour about 2 ounces concentrate into each glass; top with tonic water and stir.