Rub oil over the corn, then place on a rack about 10 cm(4 in) above the coals. Cook, turning regularly, until golden brown. Meanwhile, place butter in a small pan on barbecue and heat until it turns nut-brown. Stir in coriander, remove from heat, then stir in onion. Keep warm. Spoon over cooked corn, sprinkle with salt and pepper and eat before it goes cold.