In a small saucepan, add the water, sugar, juniper berries, grapes, orange zest and the rosemary sprig and bring to the boil over moderately high heat. Set the stove top temperature to moderately low and cook for 15 minutes or until the mixture is slightly syrupy. Stir it often once so that the grapes burst open more easily. Let cool down completely. Discard the orange zest and sprig of rosemary. Pulse coarsely chop in a mixer / blender or in a food processor. Press the mixture through a fine sieve and collect the liquid. Dispose of the residue. Let it steep in the refrigerator for at least 8 hours.
To serve, add 60 ml of this grape juice to each glass filled with ice cubes and - in sequence - add 120 ml of tonic water, 60 ml of sparkling water and 1 tablespoon of orange juice. Garnish with the sprigs of rosemary.