Preheat the oven to 200°C.
Place a baking tray in the oven to heat.
Oil four pieces of greaseproof paper and sprinkle with lime zest.
Top paper with mixed herbs and sea bass, then season with salt and pepper.
Boil lime zest, juice, 100ml water, and sugar until reduced by two-thirds into a syrup.
Cook apples with lime zest, juice, and sugar for 10 minutes until soft.
Purée apple mixture in a food processor and pass through a sieve.
Reduce fish stock by half in a saucepan over high heat.
Add cream and four lemongrass stalks, then reduce by two-thirds.
Blend cream mixture with remaining lemongrass and sieve into a pan.
Add lemon juice and seasoning to the sauce.
Bake the sea bass on the preheated tray for two minutes until cooked.
Serve fish with apple purée, lime syrup, and frothed lemongrass sauce.