Make the bumbu. Blend the shallots, garlic, chilies, ginger, turmeric, candlenuts, and coriander with 2 tablespoons water into a coarse paste.
Heat the oil in a skillet over medium. Add the paste along with the bruised lemongrass and torn makrut lime leaves. Fry, stirring often, until the paste darkens and smells toasty and the raw shallot smell is gone, 6 to 8 minutes. It is ready when the oil beads out at the edges.
Stir in the tomato, brown sugar, and salt. Cook until the tomato breaks down and the paste is jammy, about 3 minutes. Take off the heat and let it cool slightly.
Pat the fish fillets dry. Coat each one all over with the paste, then scatter the basil leaves on top.
Lay each fillet on a piece of banana leaf (softened over a flame first) or parchment or foil. Tuck a length of lemongrass alongside and fold into a sealed parcel.
Steam the parcels over simmering water for 15 minutes, or bake at 400F (200C) for 18 to 20 minutes, until the fish flakes easily.
For a charred edge, open the parcels and slide them under the broiler or onto a dry grill pan for 2 to 3 minutes. Rest 2 minutes, then open at the table and squeeze lime over the top.