Place the sliced onions, celery, carrot, parsley sprigs, and leek in the bottom of a fish cooker or a braising pan with the butter, and let simmer over a medium flame until the onions are wilted. Rub the halibut with about 2 teaspoons salt and with pepper, and place it on the bed of vegetables. Add wane to half the depth of the fish, then add the thyme and bay leaf and let it just come to a boil. Cover the fish with a piece of well-buttered parchment or brown paper and place in a 425°F oven. Cook about 20 to 25 minutes, or until the fish is just barely cooked through. While the fish is cooking, brown the onions in half the additional butter and let them cook through in a covered pan with the addition of a little white wine. Sprinkle them lightly with granulated sugar and shake the pan well while they caramelize a bit. Sauté the mushrooms lightly in the remaining butter and season to taste. They and the onions should be ready when the fish is done. Remove the fish from the pan and arrange on a hot platter, surrounded by the mushrooms and the onions. Strain the sauce and thicken it with beurre manié. Taste for seasoning and correct it. Serve the sauce separately, and garnish the fish with chopped parsley. Plain boiled potatoes and white wine go extremely well with this.