Add water, sugar, and thyme sprigs to a small saucepan. Heat until sugar dissolves.
Remove from heat and let steep for 15 minutes. Strain and cool completely.
Add one thyme sprig and syrup to the shaker. Then, muddle gently to release the oils.
Pour in the gin and fresh lemon juice. Fill the shaker with ice and shake vigorously for 10-12 seconds.
Double strain into a chilled cocktail glass. Top with a splash of club soda if desired.
Garnish with a fresh thyme sprig and serve immediately.
Notes
Store extra thyme simple syrup in an airtight container in the fridge for up to 2 weeks.Fresh lemon juice makes a big difference - avoid bottled juice.For a stronger thyme flavor, let the syrup steep longer or add more sprigs.A high-quality London dry gin works best in this recipe.Chill the cocktail glass in the freezer for 10 minutes before serving for best results.