Preheat the oven to fan 180C/conventional 200C/gas 6. Wash the sea bass inside and out, and pat dry with kitchen paper. Score across the fish and through the skin 4-5 times on each side, then lay the fish on a large piece of oiled foil, big enough to wrap it up loosely.
Put the lemongrass, chillies, garlic and ginger into a mortar with the honey and 1 tbsp of the oil. Squeeze in the juice of one of the limes. Bash a few times with the pestle until everything’s bruised – there’s no need to grind it finely.
Season the fish inside and out. Scrape out half the pounded mixture over the fish, add the last of the oil and rub everything in, making sure you push some into the cuts.
Cut the second lime into quarters and push two pieces into the cavity along with the remainder of the pounded mixture. Fold each kaffir lime leaf in half and tear out the central vein to give you two halves. Push these into the cavity, too.
Squeeze the juice from the last two lime quarters over the fish and pull the sides of the foil up to create a loose parcel. Crimp the edges to seal, making sure there is some space around the fish.
Bake for 25 minutes. If you’re eating the fish straight away, let it rest for about 5 minutes before opening the parcel. If you’re taking it to a picnic, wrap it in another layer of foil the minute it’s out of the oven.