Add gin, lemon juice and simple syrup to a cocktail shaker with ice. Shake vigorously.
Strain into a champagne flute, top with champagne. Garnish with a twist of lemon.
Notes
To make simple syrup - combine equal parts granulated sugar and water. Bring to a simmer, whisking until sugar is fully dissolved. Remove from heat and cool. Store in the fridge in a sealed container for up to two weeks.