Fill a cocktail shaker halfway with ice, then add gin, limoncello, lemon juice, and simple syrup.
Shake for about 15 seconds until the shaker is cold.
Strain into a chilled highball glass filled with fresh ice.
Top with club soda or sparkling water.
Gently stir with a bar spoon.
Garnish with a lemon slice and optional thyme sprig. Serve immediately.
Notes
Use fresh lemon juice for best results. Adjust sweetness with more or less simple syrup. For a lighter version, reduce gin and increase sparkling water. Thyme adds a pleasant herbal note but is optional.