Chill your glass. Pop a Champagne flute or coupe into the freezer for a few minutes so the drink stays cold and crisp.
Shake the base. In a shaker with ice, add gin, fresh lemon juice, and simple syrup. Shake hard for about 10–12 seconds until well chilled.
Strain. Double strain into your chilled glass to keep it silky and free of ice shards.
Top with bubbles. Gently pour chilled Champagne or dry sparkling wine over the shaken base.
Aim for a soft pour to preserve carbonation.
Garnish. Express a lemon twist over the top to release the oils, then drop it in or rest it on the rim.
Serve immediately. This cocktail is at its best the moment it’s made—bright, cold, and lively.