Prepare the Glass: Fill a rocks glass with ice cubes.
Shake the Ingredients: In a cocktail shaker, combine Hills and Harbour Gin, a squeeze of grapefruit juice, elderflower cordial, and simple syrup (if using). Fill the shaker with ice cubes.
Shake Well: Shake the ingredients vigorously for about 10-15 seconds to chill the mixture.
Strain into the Glass: Strain the cocktail into the glass using a Hawthorne strainer to catch any ice shards.
Garnish: Add the slice of grapefruit and sprig of fir to the glass.
Serve: Present this zesty, aromatic taste of the Scottish Lowlands and enjoy.