In a medium saucepan, combine equal parts honey and water, adding fresh rosemary. Heat over low to medium heat, stirring gently until honey dissolves, about 5 minutes. Allow to simmer for 10 minutes, then cool and strain into a jar.

While syrup cools, squeeze fresh grapefruit juice until you have about ½ cup. Strain through a fine mesh sieve to remove pulp or seeds.

In a tall glass, add 2 to 3 tablespoons of rosemary syrup. Pour in freshly squeezed grapefruit juice and stir gently to combine.

Fill the glass with sparkling water, pouring slowly to maintain fizz. Stir lightly to mix.

Garnish with a sprig of fresh rosemary and a slice of grapefruit. Serve over ice.