Put the ginger, water, sake, mirin, soy sauce and sugar in a frying pan and bring to the boil.
Add the flounder fillets and simmer.
Turn them over, and they are done when the seasonings have soaked into the fish.
Story Behind this Recipe
My mother told me that I had to use a large saucepan when making simmered dishes, but I had trouble when cooking fish, because I couldn't turn it over easily. I thought it would be better in a frying pan, after all, a frying pan is large and wide and the fish won't burn. I tried it and it worked fine. Now, I always make fish dishes in the frying pan.
It can be tricky to turn the fish over, so I used a spatula. It's easier if you use the edges of the pan to help you. The fish won't burn in a non-stick frying pan, so even beginners will find this easy. The fish will crumble if you leave the heat on high, so be careful.