Blanch the tomatoes in order to remove the skins. To do so first bring a medium-sized pan of water to a boil.
Fill a large bowl half full with ice water and place it in the sink.
Meanwhile, cut an x in the skin in the bottom of each tomato, being careful not to cut into the flesh.
Add the tomatoes a few at a time to the boiling water, cooking only 1 - 1˝ min, or until the skin begins to curl where you've cut the X.
Remove them from the boiling water with a slotted spoon and immerse them into the ice water immediately.
Repeat this process until all of the tomatoes have been blanched.
When they are cool enough to handle peel and dispose of the skin. Core and cut the tomatoes into 1-inch cubes; set aside.
Peel and coarsely chop the onions; set aside.
Clean and coarsely chop the mushrooms, set aside.
Preheat the oven to 350°F.
Gently rub both sides of the fish with salt and pepper; set aside.
In a large skillet, heat the olive oil over medium-high heat.
Add the onions and sauté, stirring frequently, for 8 to 10 minutes until the onions begin to color.
Spoon the onions evenly over the bottom of a baking pan large enough to hold the filets in a single layer.
Place the fish over the onions, scatter the tomatoes and mushrooms over the fish, then pour the wine over all.
Bake, basting occasionally, for 15 to 20 minutes until a skewer inserted in the thickest part of the fish meets with no resistance.
Serve some of the vegetables and pan juices with each portion; sprinkle with parsley.
