Make the thyme simple syrup: Combine water and sugar in a small pot. Heat until sugar dissolves. Add thyme sprigs and let steep for 15 minutes. Strain and cool.
Juice the lemons and strain out pulp and seeds.
In a large pitcher, mix lemon juice with the cooled thyme simple syrup.
Add sparkling water and stir gently to combine.
Fill Collins glasses with ice cubes.
Pour the mixture into glasses.
Garnish each glass with a fresh thyme sprig and lemon wheel.
Notes
The thyme simple syrup stays fresh in the fridge for up to 2 weeks in an airtight container.This recipe makes 4 servings. Double or triple the amounts for larger groups.For best results, use fresh lemons instead of bottled juice.The drink can be prepared up to 4 hours ahead - just add the sparkling water right before serving to keep the fizz.