Heat grill to its highest setting. Rinse the fish under running cold water to dislodge any loose scales. Brush with a little of the oil and season lightly. Grill for 6-8 mins, or until cooked; the eye should be white, the skin well browned and the flesh firm and opaque.
Meanwhile, make the dressing: whisk the mustard, basil, honey, lemon zest and juice and remaining oil together in a small bowl, and season. Once the fish is cooked, spoon the dressing over and serve.