1) Wash the fish head, split it in the middle, and use paper towels to dry the surface of the fish head. Cut the soft tofu into 1cm thick chunks. Soak the mushrooms in warm water for 5 minutes, remove the stems and wash them.
2) Pour oil into the frying pan. When it is 70% hot, add the fish head and fry on both sides over medium heat until browned (about 3 minutes per side). Place the fish head on one side of the pot, sauté the green onions and ginger slices in the oil in the pot, then pour enough boiling water to cover the fish head.
3) Add the mushrooms, cover, and simmer over high heat for 50 minutes.
4) Add salt, add tofu and continue cooking for 3 minutes.