Tear off 2 sheets of heavy-duty aluminum foil, about 10 inches long each. Lie out foil shiny-side-up. Dot each foil sheet with butter, then cover with sliced onion. Fold up edges of foil. Place one piece of fish in the center of each foil sheet. Sprinkle fish with salt and pepper, wine, garlic and remaining butter. Place sliced mushrooms and lemon on each fillet and fold foil over to seal. Bake in oven at 400 degrees F for 12-15 minutes (can also be cooked over grill or campfire). Remove and serve on plate or eat right out of the foil!
Put all ingredients into a zip-top plastic bag. Shake, remove and place on grill. Grill for about 10 minutes or until done.