1Rinse the mackerel thoroughly under cold water. Pat the mackerel very dry with paper towels. Make diagonal slits on both sides of the mackerel, about 2-3 cm apart, going about halfway through the flesh.
3While the mackerel is resting, prepare the dipping sauce. In a small bowl, combine soy sauce, minced garlic, chopped green onion, gochugaru, and toasted sesame seeds. Mix well and set aside.