This is a family favorite we had in Alaska all the time and now I do it with any white fish. However I do all sorts of variations of this now to change up the flavors.
1/4 c sour cream (I use yogurt)
1/4 c mayo
salt and pepper to taste
1. Sautee onions in oil/butter and place on bottom of buttered dish.
2. Place fish on onions.
3. Mix mayo, cream, parmesan, garlic, lemon juice and seasonings; spread on top of fish.
4. Cover dish with foil and cook 30 mins on 375. Then uncover and cook or broil a few more minutes.
My simplified version of this is to sautee onions, put fish over top, then spread minimal amount of homemade mayo/yogurt directly on top (about 2 T) and add garlic power (or other seasonings like dill). Then I pop it in the toaster oven and broil for about 10 mins total. It doesn't even need parmesan or lemon juice (depending on the type of fish).