Add the grapefruit pulp to a large pitcher and gently mash it just enough to release juice and aroma without turning it into purée, then add the honey and stir until the honey is fully dissolved and the base is uniform.
Lightly crush the rosemary sprigs between your fingers to release oils without bruising them into bitterness, then add them to the pitcher and stir gently once to distribute aroma.
Pour in the ice-cold water to reach a total volume of about 3.2 liters, add the thin grapefruit slices, and stir gently to combine without breaking down the fruit.
Refrigerate for at least 1 hour for a clean citrus-herbal profile; for deeper rosemary character, infuse up to 4 hours, but stop there — rosemary escalates fast and will dominate if left longer.
Serve well chilled, straight from the pitcher or over ice if preferred.
