Bring a pot of water to a boil. Mix the soy sauce, mirin, lemon juice, and water together. Adjust to taste. Then place in a saucepot and bring to a boil.
Quickly blanch the halibut cheeks in the hot water (30 seconds). Then place in the boiling soy sauce mixture, cover with a clean paper towel (to soak in and spread the sauce), and simmer for 4-5 minutes, until translucent in the middle.
Remove the cheeks and top with the slices of scallion.