Juice the fresh grapefruits until you have one cup, strain the juice to remove pulp. Warm the juice with grapefruit zest in a saucepan over low heat for 5 minutes. Sprinkle gelatin over the juice and let bloom for 5 minutes, then stir and refrigerate for at least 2 hours until set.

In a saucepan, combine heavy cream and milk, bring to a simmer while stirring. Remove from heat, add rosemary and steep for 10 minutes. Strain the infused cream into a bowl, mix in honey until fully dissolved.

Sprinkle gelatin over cold water and let sit for 5 minutes to bloom. Reheat cream mixture and whisk in bloomed gelatin until dissolved.

Pour the panna cotta mixture carefully over the set grapefruit jelly in ramekins, then cover with plastic wrap and refrigerate for at least 4 hours.

Dip ramekins in hot water for 10 seconds, run a knife along the edges, invert onto a plate to serve.

Garnish with fresh berries, mint, or a drizzle of honey before serving.