Peel and slice ginger thinly. Cut white long onions into 2" lengths. Blanch fish fillets in boiling water for a couple of seconds. Immediately transfer to ice water. Remove after cooled, and pat dry with a paper towel.

In a medium pot, put water, Sake, Soy Sauce, Mirin, and sugar, and let them boil. Add ginger, onions, and fish into the pot. Place parchment paper over the fish, and cook about 10 minutes.