Begin by juicing the grapefruit and zesting its skin, aiming for about 1 cup of juice. In a small saucepan, gently heat the juice over low heat for about 5 minutes, just until warm. Bloom the gelatin by mixing it with a couple of tablespoons of cold water in a bowl, then stir this mixture into the warm juice until fully dissolved. Pour the mixture into ramekins and refrigerate for 1-2 hours, or until set.

In a saucepan, combine the heavy cream and milk, and heat them over medium heat until steaming—don't let it boil, which takes about 5 minutes. Add the fresh rosemary, allowing it to steep for 10 minutes. After steeping, strain the mixture to remove the rosemary, then return it to the heat. Bloom the gelatin in a separate bowl with cold water, then stir it into the warm cream mixture along with the honey until completely dissolved.

Gently pour the panna cotta mixture over the set grapefruit jelly in the ramekins, filling to the top. Ensure the panna cotta is poured slowly to avoid disturbing the jelly. Once all ramekins are filled, cover them with plastic wrap and refrigerate for at least 4 hours, but for the best results, let them set overnight to achieve that perfect silky texture.

When ready to serve your rosemary honey panna cotta, remove the ramekins from the refrigerator. Briefly dip the bottoms of the ramekins in hot water for about 10 seconds to loosen the panna cotta. Run a knife around the edges carefully, then invert each ramekin onto a dessert plate. Allow a gentle tap on the bottom to help it release, and serve with additional grapefruit garnish if desired.