½ fluid ounce lemon juice
5 fluid ounces brut champagne, chilled
Chill cocktail shaker and glass (champagne flute or collins glass) in the freezer.
Pour gin, simple syrup, and lemon juice into shaker. Shake well.
Fill chilled glass half full of ice, then strain cocktail into the glass.
Top with champagne and garnish with a lemon slice.