Add water and sugar to the saucepan. Bring to a simmer over medium heat, stirring often until the sugar is dissolved.
Once the mixture simmers, add the lavender and basil leaves.
Lower the heat and let it simmer for 2-3 minutes, giving it a stir now and then.
Remove from heat. Let the syrup steep for about 10 minutes.
Strain the syrup through the fine mesh strainer into a clean jar or bowl.
Fill a serving glass with ice cubes.
Add 2–3 tablespoons of lavender basil syrup to the glass.
Squeeze in the lemon juice for a bit of brightness.
Top with 1 cup of club soda or sparkling water and gently stir.
Garnish with fresh basil leaves and lemon slices if you’re feeling fancy.