How to Make It
In a saucepan, cover the turnips with water and bring to a boil. Add the ginger and simmer over moderately high heat until the turnips are tender, about 20 minutes; drain, reserving 1/4 cup of the cooking liquid and the ginger. Transfer the turnips, ginger and cooking liquid to a blender, add the butter and puree until very smooth. Season with salt and white pepper and keep warm.
In a saucepan, combine the fish trimmings with the water, sake, garlic, ginger and scallion and bring to a boil. Simmer over low heat for 5 minutes. Strain the broth through a fine strainer and return it to the saucepan. Whisk in the miso and 1 teaspoon of the soy sauce and season with salt and pepper.
In a small skillet, combine the mushrooms with 2 tablespoons of the sake broth and the remaining 1 teaspoon of soy sauce. Cover and cook over moderate heat until the mushrooms are tender, 2 minutes.
Preheat the oven to 400°. In a large ovenproof skillet, heat the oil. Season the monkfish with salt, white pepper and the Sichuan peppercorns and dust with flour. Add the fish to the skillet and cook over high heat until golden on 3 sides, about 6 minutes total. Transfer the skillet to the oven and roast the fish for about 5 minutes, until a metal skewer inserted in the center feels warm to the touch. Transfer the monkfish to a carving board and let rest for 5 minutes.
Slice the fish 1/3 inch thick. Reheat the sake broth and spoon it into shallow bowls. Spoon the turnip puree into the bowls, top with the fish and mushrooms and serve.