Put the garlic, shallots, lemon grass, turmeric, ginger, chilli and salt and pepper into a food processor or blender and process until a paste is formed, adding the oil and fish sauce to help the grinding.
Place the fish in a bowl and toss with the spice paste. Cover and refrigerate for 15 minutes.
Thread the pieces of fish on to skewers and arrange on a foil-lined tray. Cook under a preheated hot grill for 4–5 minutes, turning once so that the pieces brown evenly. Serve sprinkled with the coriander.