Combine the light brown sugar and water in a small saucepan and heat gently over medium heat, stirring continuously until the sugar fully dissolves into a smooth syrup. Do not allow the mixture to boil or reduce. Remove from heat and immediately add the rosemary sprigs and the strip of pink grapefruit peel. Let the aromatics steep for about 6–8 minutes only — just long enough to extract herbal oils and citrus brightness without introducing bitterness or resinous notes. Strain out the solids and allow the syrup to cool completely before use.

In a large pitcher, combine the fresh pink grapefruit juice with about 80–100 ml of the cooled rosemary grapefruit syrup. Stir slowly until fully integrated. Taste carefully — the liquid should feel bright, lightly sweet, and pleasantly bitter with a clean citrus finish. Adjust in very small increments if needed, keeping the overall profile dry and refreshing rather than sweet.

Fill the pitcher generously with ice to initiate chilling and gentle dilution. Pour in the chilled club soda last and stir once or twice only. Overmixing will release carbonation and flatten the drink’s crisp texture, which is essential to its Paloma-style character.

Pour into ice-filled glasses and garnish with fresh rosemary sprigs and pink grapefruit slices. Serve fully cold while bubbles are lively and aromatics are at their peak.