Make the simple syrup: Combine water and sugar in a saucepan. Heat until sugar dissolves. Add blueberries and sage. Simmer 5 minutes. Strain and cool.
Add blueberries and sage leaves to the shaker. Muddle gently to release flavors.
Pour in simple syrup and lemon juice. Fill shaker with ice.
Shake for 15 seconds until well chilled.
Strain into a tall glass filled with ice.
Top with sparkling water and stir gently.
Garnish with fresh blueberries and a sage leaf.
Notes
Store extra simple syrup in an airtight container in the fridge for up to 2 weeks.Fresh blueberries work best, but frozen can be used in a pinch.For a sweeter drink, add more simple syrup.Replace sparkling water with ginger ale for extra flavor.Make it ahead: Prepare the simple syrup and store separately from other ingredients.