Pat the yellowtail fillets dry with a paper towel.

In a storage bag, combine sake, mirin, sugar, and soy sauce. Mix well by massaging the bag.

Add the yellowtail fillets to the bag, ensuring they are well-coated with the marinade. Remove excess air and seal the bag.

For freezing: Lay the bag flat on a tray and freeze. For refrigeration: Store in the fridge and use within 3 days.

To cook from frozen: Defrost the fillets using the microwave defrost function or by natural thawing in the fridge.

Heat salad oil in a frying pan over medium heat. Drain excess marinade from the fillets and place them skin-side down in the pan.

Cook for about 3 minutes until the skin is golden brown. Flip the fillets and cook for another 2 minutes until fully cooked.