In a small saucepan, combine the grapefruit juice and honey (or agave syrup) over medium heat. Stir until the honey is fully dissolved, creating a grapefruit syrup. Remove from heat and let cool.
Once the syrup is cooled, take the fresh rosemary sprig and lightly bruise it by tapping it with the back of a knife or rolling pin to release the oils.
In a large serving pitcher, combine the grapefruit syrup, bruised rosemary, and sparkling water. Stir gently to mix everything without losing too much carbonation.
Fill glasses with ice cubes and pour the spritzer over the ice.
Garnish each glass with a slice of grapefruit and a sprig of fresh rosemary for a beautiful presentation.
Notes
For a sweeter drink, adjust the amount of honey or agave syrup to taste.