Preheat the oven to 200°C.
Dissolve sugar in vinegar over medium heat, then simmer with onion, mango, and ginger for 6 minutes.
Stir in the chilli and set the mixture aside to cool.
Season cod with salt, pepper, and curry powder.
Fry the cod skin-side down in butter and oil for 4 minutes until crisp.
Divide mango mixture onto four parchment sheets and top with cod skin-side up.
Fold and seal the parchment securely to create four parcels.
Bake the parcels on a tray for 10 minutes and rest for 2 minutes.
Boil fish stock and coconut milk in a pan and reduce by half.
Whisk parcel juices, whipped cream, chervil, and lemon juice into the sauce.
Serve fish with the sauce, lettuce, and cucumber.