Preheat oven to 375°F
Slice onion and place in 13x9 baking dish
Pour melted butter over onion
Place halibut in baking dish, pepper to taste
Combine and mix sour cream, mayonnaise, green onion and parmesan chees and spread over Halibut.
(optional) Sprinkle bread crumbs over dish.
Place the casserole dish in the preheated 375* oven and bake for 30 minutes or until the halibut reaches 140 ˚F. (Broil if necessary to brown the topping to your liking)