Bring water to a boil in a pan. Prepare ice water in a bowl. When boiled water, turn off the heat and blanch the flatfish for 10 seconds and immediately plunge into the ice water. Pat them dry with paper towels.
Combine the sauce ingredients (*) in a pan, bring to a simmer over medium-high heat. Add the flatfish and cook for 5 minutes with a small lid directly on the flatfishes (you can substitute with aluminum foil cut into the size of inner pan).
Add the ginger and keep cooking with the same lid for 8~10 minutes. Skim off the foam when it rises on the surface.
※To your taste, after removing the flatfish, simmer the sauce until it thickens a little and pour the sauce over the flatfishes.