In a medium saucepan, bring the water and agave nectar to a boil until the mixture bubbles and is slightly syrupy. Remove from the heat; add the lemon zest and lemon juice and stir well. Transfer to a medium bowl and set aside to cool completely. If you want to speed up cooling, place the bowl in an ice bath.
In the meantime, puree strawberries in a blender or immersion hand blender. Add the pureed strawberries to the cooled lemon juice. Stir well to combine and strain using a fine mesh strainer. Taste and adjust with additional agave nectar if needed. Pour into a pitcher and place in the refrigerator to cool completely.
To serve:
Add the sparkling water and stir well. Pour over glasses filled with ice and serve, garnished with strawberries.