Place 2 yellowtail fillets in a sealable bag and add 1 tsp soy sauce and 1 tsp mirin. Seal the bag and marinate in the refrigerator for 15 minutes.
In a small bowl, mix 2 tbsp soy sauce, 2 tbsp mirin, 2 tbsp sake and 1 tsp sugar until the sugar has dissolved. Set by the stove for later.
Once 15 minutes have passed, transfer the yellowtail fillets to a container and pat them dry with kitchen paper.
Sprinkle with 1 tbsp cornstarch, making sure both sides are lightly covered. Pat off the excess before frying.
Heat a pan on medium and add ½ tbsp cooking oil. Once hot, place the fillets in the pan and fry for 2 minutes on each side.
Increase the heat to high and pour the teriyaki sauce into the pan and fry until thickened to a syrup-like consistency. Turn the fish to make sure both sides are covered with sauce (about 30 seconds each side).
Once thickened, remove the pan from the heat and serve.
Enjoy!