Fill a cocktail shaker halfway with ice cubes.
Add the gin, fresh lemon juice, orange liqueur, grenadine, and simple syrup to the shaker.
Shake vigorously for 10–12 seconds until well chilled.
Strain into an old-fashioned glass over fresh ice.
Garnish with a maraschino cherry and an orange slice.
The Daisy is one of the oldest cocktail templates - spirit, citrus, and a sweetener with character - and grenadine is what distinguishes a Gin Daisy from a simple Gin Sour; don't substitute simple syrup or you'll lose that rosy depth. For best results, use a London Dry gin with good botanical presence to stand up to the citrus.
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