If you can, I highly recommend getting all the ingredients prepped before you start - Mise en Place. This way you're not scrambling around chopping something while the rest of the meal is overcooking.
Start out by finely chopping the onion and slicing the mushrooms. Besides having all the other ingredients available, chop up the parsley.
(1) preheat your oven to 450°F. I made the mistake of waiting until I needed the oven and it took a lot longer than expected to reach the right temperature.
(2) Heat the olive oil in your pan over medium high heat and sauté the chopped onion until it's translucent.
(3) Deglaze the pan with Marsala wine. Be careful to remove the pan from the stove when doing this to prevent the wine from igniting in your face. You can use white wine if you don't have any Marsala wine. It will give the dish a slightly different taste, but you may like it better.
(4) When most of the wine is cooked off, add the mushrooms and butter. This recipe would have a lot more flavor if you were to use wild mushrooms, but at the time, all I had were plain old boring white mushrooms and it still came out great with lots of flavor.
Reduce the heat to medium and cook until the mushrooms are tender. How do you know when they are tender? Taste one! That's part of the enjoyment of cooking, you get to taste as you go along. It's also a great way to learn what works and what doesn't. If you just follow a recipe without ever tasting, you'll never learn the effects ingredients have on a dish.
This is especially true with salt. I've made soups that tasted OK but after adding a little salt, it had a wonderful new flavor. So make sure you taste as you go along.
(5) At this point add the chicken stock, a little salt and pepper, and let the sauce cook down until it thickens a little. Rule of thumb: when the sauce can coat a spoon, it is the correct thickness. This is something you need to play around with until you learn to get it to the thickness you like.
(6) In an oven proof sauté (fry) pan, heat the canola oil until its so hot it's about to smoke. Be careful around this hot oil!
(7) Season the fillets with salt and pepper and add to the hot pan. Now here is where I had a small problem. My fillets didn't have skin on them so I had to adjust my cooking times to compensate. Otherwise, I would have started cooking them skin side down for approximately 5 minutes until the skin was nice and crispy. Then I would have flipped them over for 30 seconds, transferred them into a 450° oven and roasted them for about 3-4 minutes.
(8) But since my fillets were skinless, cook them on one side for about 5 to 6 minutes and flip them over for another 2 minutes before transferring to the oven for 3 to 4 minutes. My results? Plump, moist, and tender, but you may want to experiment with these times because there are so many variables that can go into any recipe. Type of pan, thickness of fish, stove's btu's, oven temperature, pan temperature, type of oil. All of these factors go into cooking times and cannot be accounted for in a recipe. Using your senses and experience are vital for any recipe to work.
(9) On warm plates ( I usually heat them in the microwave for about 2 minutes), dish out the onion-mushroom mixture and top with the pan roasted fillets. Sprinkle a little of the chopped parsley and serve.
I served this meal with wild rice, green beans, and a wonderful Italian Orvieto white wine. The sea bass combined with the onion-mushroom mixture worked out better than I expected. And my wife loved it. Enjoy!