4 tablespoons unsalted butter
1/2 pound shiitake mushrooms, stems discarded, caps cut into 1/4-inch dice
Salt and freshly ground pepper
1/4 cup dry white wine, such as Riesling
2 pounds slender leeks, white and tender green parts only, halved lengthwise and cut crosswise on the diagonal 1/2 inch thick
1 cup chicken stock or canned low-sodium broth
1 1/2 pounds trimmed monkfish fillet, cut crosswise into 1/2-inch-thick medallions
Unbleached all-purpose flour, for dredging
1 tablespoon vegetable oil
2 tablespoons minced chives or a combination of chives and garlic chives
Melt 1 tablespoon of the butter in a very large skillet. When the butter starts to brown, add the mushrooms, season with salt and pepper and cook over moderate heat, stirring, until softened, about 3 minutes. Add the wine and cook until evaporated, about 1 minute. Transfer the mushrooms to a bowl and wipe out the skillet.
In a medium enameled cast-iron casserole, melt 2 tablespoons of the butter. Add the leeks, season with salt and pepper and cook over moderate heat, stirring, until softened, about 1 minute. Add the chicken stock and bring to a boil. Cover and cook over moderate heat for 3 minutes. Stir well, cover and cook for 3 minutes longer.
Season the monkfish medallions with salt and pepper and lightly dredge them in flour; shake off the excess flour. In the very large skillet, heat the vegetable oil until almost smoking. Add the monkfish medallions and cook them over moderately high heat until browned on the bottom, about 3 minutes. Turn the medallions, reduce the heat to moderate and cook until just opaque throughout, about 3 minutes.
Bring the stewed leeks to a boil. Stir in the shiitakes and season with salt and pepper. Remove from the heat and stir in the chives and the remaining 1 tablespoon of butter. Spoon the leek stew onto plates, set the monkfish on top and serve at once.