Put the amberjack pieces in a non-stick frying pan. Add mentsuyu sauce, mirin and cooking sake and turn on the heat. (No oil needed.)
Simmer over medium heat with a lid on until the sauce is bubbling. Continue simmering while spooning the sauce over the fish. If it looks like the sauce is getting too thick, add a little water.
Add just a little more mirin at the end to make the fish really glossy.
Story Behind this Recipe
I didn't want to bother defrosting amberjack that I had in the freezer, so I tried steaming it in the sauce and it was delicious. (If using frozen fillets, leave the lid on for longer.) Room temperature fish fillets worked well too!!
If the heat is too high the sauce will thicken too fast, so simmer throughout over medium-low heat and you won't mess up.