To make the marinade, combine the soy sauce, sesame oil, garlic and ginger and mix well. Rinse the mackerels (inside and out) in cold water, pat dry, coat with the marinade inside and out. Brush the corn cobs with a little marinade. Cover the fish and marinate in the fridge for approx. 1 hr. Cover the sweetcorn and remaining marinade, set aside. Remove the fish from the fridge approx. 30 mins. before cooking.
Mix the spring onions with the sesame oil, divide between the fish. Place the fish in greased grill baskets, top with slices of lemon and seal the baskets.
Gas/electric grill: Grill the fish and corncobs over/on a medium heat for approx. 20 mins. Brush with the reserved marinade from time to time, turning the fish once and the corn cobs several times.