Bring 1/4 cup sugar, 1/4 cup water and 2 sprigs rosemary to a simmer in a small saucepan, stirring until the sugar dissolves, about 2 minutes.
Take the pan off the heat and let the rosemary steep while the syrup cools, at least 20 minutes. Remove the rosemary. This makes enough syrup for about 4 cocktails.
The syrup keeps for 2 weeks in a jar in the fridge. Make it ahead and the cocktail takes 2 minutes.
Cocktail
Fill a cocktail shaker with ice. Add 2 oz white rum, 3 oz orange juice, 1/2 oz lime juice, 3/4 oz of the rosemary syrup and 2 dashes orange bitters. Shake hard for 15 seconds.
Strain into a copa glass filled with fresh ice and top with 2 oz soda water.
Clap 1 sprig rosemary between your hands to release its aroma and tuck it into the glass with 1 slice orange.
Notes
For a non-alcoholic version, leave out the rum and top the glass with extra soda water.Blood oranges, when they are in season, turn this into a ruby-red drink.