In a small saucepan, combine equal parts honey and water with a handful of fresh rosemary sprigs. Heat over medium, stirring gently until the mixture comes to a simmer. Allow it to steep for about 15 minutes, then remove from heat and let it cool completely.

Juice the grapefruit by cutting them in half and extracting the juice until you have about a cup of fresh juice. Strain the juice through a fine mesh sieve to remove any pulp and seeds.

In a large glass or pitcher, combine the cooled rosemary simple syrup with the freshly squeezed grapefruit juice. Stir until well mixed, adjusting sweetness by adding more syrup if needed. Finally, pour in the chilled sparkling water.

Fill tall glasses with ice and pour the prepared soda over the top. Garnish each glass with a sprig of rosemary or a slice of grapefruit. Serve immediately.