Juice the grapefruits to yield about 1 cup of juice. In a small saucepan, combine the grapefruit juice with the zest over low heat and warm gently for about 5 minutes. Strain the mixture to remove zest, and sweeten to taste if desired.

Soak the gelatin in cold water for about 5 minutes until softened, then stir it into the warm grapefruit juice until fully dissolved. Pour the mixture into ramekins and refrigerate until set, approximately 1-2 hours.

In a saucepan, heat the heavy cream and milk over medium heat until steaming but not boiling, about 5-7 minutes. Remove from heat and add chopped fresh rosemary; let steep for 30 minutes. Strain to remove rosemary.

In the meantime, bloom the gelatin in cold water for 5 minutes. Stir honey into the warm rosemary-infused cream and add bloomed gelatin, mixing until fully dissolved.

Carefully pour the creamy rosemary mixture over the jelly in the ramekins, filling them to the top. Cover loosely with plastic wrap and return to the fridge to set for at least 4 hours, preferably overnight.

To serve, dip the ramekins briefly in warm water for 5-10 seconds to loosen the edges. Run a knife around the inside edge and flip onto a plate. Garnish with fresh herbs or edible flowers.