How to Make It
In a medium saucepan, combine half of the chopped rhubarb with 1/2 cup of the sugar and 3/4 cup of water. Simmer over moderate heat until the syrup is deep pink, about 15 minutes. Remove from the heat and let steep for 1 hour. Strain the syrup through a fine-mesh sieve into a small bowl, pressing on the solids, then return the syrup to the saucepan. Discard the solids.
Lightly grease six 6-ounce ramekins. Fill a large bowl with ice water. In a small bowl, sprinkle the gelatin over 1/2 cup of the cream and let stand until softened, 5 to 10 minutes.
Add the remaining 3 1/2 cups of cream and the vanilla seeds to the rhubarb syrup and bring to a simmer over moderately low heat; do not boil. Stir the softened gelatin into the saucepan until dissolved. Transfer the mixture to a medium bowl and set the bowl in the ice bath, stirring occasionally, until the custard is cool and begins to thicken, 15 to 20 minutes. Pour the mixture into the prepared ramekins and refrigerate until set, at least 4 hours.
Meanwhile, in a small saucepan, combine the remaining chopped rhubarb with the remaining 1/3 cup of sugar. Add 3/4 cup of water, the ginger preserves, lemon zest and lemon juice. Cook over moderate heat, stirring, until the rhubarb is justsoftened, about 5 minutes; strain the syrup and reserve the poached rhubarb and ginger for garnish. Return the syrup to the saucepan and boil over moderately high heat until thickened and reduced by half, about 10 minutes. Transfer to a bowl; let cool. Stir in the mint.
Run a knife around the inside of each ramekin. Invert each panna cotta onto a plate. Spoon some poached rhubarb and ginger around each panna cotta, drizzle with the syrup and serve.