By any chance, was the dish wrapped in paper? That's a method the French call "en papillotte
". You would season the fish, lay it in the paper, sprinkle with onions and tomatoes, dribble a little bit of wine over it, maybe a pat of real, unsalted butter, fold/crimp the paper to seal it, then bake for about 20 minutes in a 350 degree F oven.
You could also do a saute/broil type thing. Heat your broiler. Get all your food ingredients ready. You want to use a pan that is safe to go from the stovetop to the broiler. Cast Iron skillets are great for this. You want the pan to be a little bigger than the fish being cooked. A 10" pan for a couple halibut steaks is probably just right. Once the broiler is hot, place a pan on the stove and get it hot. Season your halibut. Melt a little clarified butter (canola oil will do) in the pan. Add the fish. Cook until it's seared and lightly caramelized. Flip the fish, and add some onions and tomatoes around the fish. Place the whole thing in the broiler, close enough to the element that the fish is nicely browned and fully cooked into about 5 - 7 minutes. Remove the pan from the broiler (be sure to use a hot pad). Using a slotted spatula, remove the fish from the pan and plate it. Splash just a bit of wine into the pan, off the heat, and swirl it around for a second to deglaze. Add a tablespoon or two of cold, whole butter. Swirl this until the butter melts and begins to thicken the "sauce". Drizzle this over the fish, with the onions and tomatoes.
You could just bake the whole thing. Get a baking dish, say, an 8 x 13" casserole. Either rub the dish with oil, or spray some Pam into the pan. Lay the fish in, and scatter the tomatoes and onions around. Sprinkle some salt and pepper (or seasoning mix of your choice) over it all. Splash a little bit of wine into the dish, and wrap with foil. Bake @ 350 degrees for 15 minutes, or until the fish is done.
Personally, I would go with shallots instead of onions. Shallots have a better flavor for this kind of thing, but they are hard to find. Reasor's should have them, and I know Nam Hai (Asian grocery store at 21st and Garnett) does. As far as seasoning, I mentioned "salt and pepper". You can, however, use a little Old Bay seasoning, by itself or with the S&P, or a different mix that you happen to like.
Hopefully, that helps. If you have any other questions, feel free to ask.
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