Using a medium saute pan, heat the oil over medium heat. Add the onion and garlic and cook until it is tender and translucent, but not browned. About 10 minutes.
Grease the bottom of a 12x8-9nch baking dish.
Cover the bottom of the baking dish with the onions.
Using a medium bowl, mix the mayonnaise with the sour cream.
Put the halibut filets on top of the onions in the baking dish and season with salt and pepper to taste.
Spread the top of the filets with the mayonnaise mixture.
Using a medium bowl, mix the bread crumbs with the melted butter and parsley.
Sprinkle the bread crumb mixture over the mayonnaise.
Bake at 375 °F for about 30 minutes, until the top is golden and it begins to bubble a bit.