-buttered * BOUQUET GARNI In a piece of cheesecloth, tie the following: 3 to 4 sprigs of parsley, 1 bay leaf, 1/2 celery stalk with leaves, 1/2 t dry thyme. Leave the string long enough to hang the garni in the pot and tie the string to the pot handle for easy removal. ** Beurre Manie is used to thicken sauces. Combine 1 T flour and 1 T sweet butter on a small flat plate and blend the mixture with a fork into a very smooth paste. Roll the paste into a ball. Wipe the mushrooms with damp paper towels. Remove the stems and add them to the fish stock. Heat the stock, simmer it for 15 minutes and reserve. Poach the olives in boiling water over low heat for 5 minutes. Drain on paper towels and set aside. Dip the halibut steaks into milk for 2 or 3 minutes. Dry them with paper towels. Season the fish steaks with salt and pepper and lightly dredge them in flour, shaking off the excess. In a large skillet melt the butter and 2 T of oil. Add the fish steaks and brown them well on one side. Turn them carefully with a large spatula and brown them on the other side. Carefully remove the fish to a side dish. To the fat remaining in the pan add the shallots. Cook over low heat until they are soft but not browned. Add the tomatoes, salt, pepper, Bouquet Garni and oregano. Bring the mixture to a boil and add tomato paste and the fish stock. Return the fish steaks to the skillet. Cover it tightly and simmer on top of the stove for 20 to 25 minutes, or until the fish flakes easily. Carefully remove the steaks to a serving platter. While the fish steaks are cooking, heat remaining oil in a skillet. Add the mushrooms and cook them over high heat until they are nicely browned. Do not overcook. Remove the skillet from the heat and reserve. When the fish steaks are done, remove them carefully to a serving platter. Strain the sauce, pressing down well on the tomato pulp to extract all the juice. Return the sauce to the pan and reduce it by 1/3 over high heat. Add a Beurre Manie, a little at a time, whisking constantly until the sauce heavily coats a spoon. Add the mushrooms and olives, salt and black pepper. Remove the sauce from the heat. Remove any accumulated juices around the fish steaks with paper towels. Pour the sauce over the fish steaks, and keep them warm until serving time. Combine the parsley, garlic and lemon rind and sprinkle the mixture over the fish steaks just before serving. Serve the fish steaks with the optional buttered new potatoes.