Make a fish stock. Melt 1/2 cup of the butter in a stockpot and cook the onions until softened, about 5 minutes. Add the fish bones, bouquet garni, salt and pepper. With a sharp knife cut away the skin and pith from a 1/2 lemon and discard it. Add the lemon flesh to the pot, along with 3 cups water. Season with salt and pepper. Preheat the oven to 350 F. Butter a roasting pan and sprinkle it with the shallots. Place the monkfish steaks on the shallots. Squeeze the remaining 1/2 lemon over the fish steaks and pour in the wine and strained fish stock. Bring to a boil then place the pan in the oven immediately. Cook for 10-15 minutes. Arrange the leeks in a buttered gratin dish. Remove the monkfish from the pan and place on the leeks in the gratin dish. Remove the string from the steaks. Reduce the cooking liquid from the roasting pan until there is only about 1/4 cup. Stir the crÅme fraöche into the reduced cooking liquid and season with a pinch of cayenne pepper. Spoon the sauce over the steaks and bake for 5 more minutes. Sprinkle with chopped parsley before serving.
