1Chill cocktail shaker and glass (champagne flute or collins glass) in the freezer. Add ice to shaker. 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.
Footnotes
Cook's Note
Don't skimp on the ice or skip the chilling. This drink was meant to be served COLD.