Begin by preparing the lavender syrup. In a small saucepan, combine the boiling water, dried lavender, and sweetener. Bring the mixture to a gentle boil over medium heat.
Once it reaches a boil, reduce the heat to its lowest setting so the mixture remains just steaming. Allow it to steep until the lavender loses its color and the liquid takes on a fragrant aroma—about 7 to 8 minutes.
Strain the syrup to remove the lavender buds and set the liquid aside to cool completely.
To assemble the drink, pour 2 tablespoons of the lavender syrup and 2 tablespoons of lemon juice into a glass.
Top off the glass with sparkling water and stir gently to combine.
Add ice cubes and garnish with a lemon slice, sprig of lavender, or fresh mint if desired. Serve chilled.