Fill a highball glass with ice.
Add 45ml gin, 30ml fresh orange juice, 30ml fresh lemon juice, and 25ml rosemary syrup directly into the glass.
Stir gently with a bar spoon to combine the base ingredients.
Top with 90ml chilled soda water and give one final gentle stir to integrate.
Express an orange peel over the glass and place it on the rim.
Lay a fresh rosemary sprig across the top of the glass to garnish.
Reduce the orange juice to 30ml to keep the classic Collins balance - too much OJ overwhelms the gin's botanicals and pushes the drink sweet. Freshly squeezed juice makes a noticeable difference here.
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