Caramelize brown sugar in a saucepan. Deglaze with lime juice.
Add passionfruit purée. Bruise lemongrass and slice ginger, then add both. Simmer for 5 minutes.
Peel and dice pineapple into small cubes (about 0.5 cm).
Strain the infusion. Return liquid to pot, add diced pineapple, and simmer again.
Strain again and reserve both liquid and fruit.
Soften gelatin in cold water. Warm reserved liquid and dissolve gelatin.
Line a 16 cm ring with plastic wrap. Distribute pineapple, then pour in the warm gel.
Freeze for 2 hours until firm.