Add gin, Grand Marnier, fresh lemon juice, and grenadine to a cocktail shaker.
Fill the shaker with ice.
Shake vigorously for 10–12 seconds until well chilled.
Double strain into a chilled cocktail glass.
Serve immediately, ungarnished or with a lemon twist.
The grenadine quantity here is notably generous at 45ml - most classic A1 recipes call for just a small dash or a barspoon, so reduce to taste if you want the lemon and Grand Marnier to stay in the foreground rather than tipping the drink sweet.
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