Pound chilli, ginger, lemongrass, garlic, makrut lime leaves, coriander roots, spring onion, palm sugar and 2 tsp sea salt with a large mortar and pestle, or blend in a blender to a paste, then add oil and mix to combine.
Slash both sides of each fish 4-5 times with a sharp knife, then rub marinade all over and inside the cavity. Cover and refrigerate for 2-3 hours to marinate. Remove from fridge 20 minutes before cooking to come to room temperature.
Heat barbecue or a char-grill pan to medium-high heat and lightly oil grill or pan. Barbecue fish until charred and just cooked through on one side (5-6 minutes). Turn and repeat. Season to taste, squeeze lemon and lime juice over, scatter with lemongrass, lime leaves, coriander and pickled chilli and serve.
