Preheat The Oven To 350 Degrees F (175 Degrees C). Grease The Inside Of A 9X13-Inch Baking Dish With Butter. Place Halibut In The Prepared Baking Dish. Mix Sour Cream, Green Onions, Butter, Salt, White Pepper, And Dill In A Bowl. Spread Sour Cream Mixture Over Halibut. Bake Halibut In Preheated Oven Until Fish Flakes Easily With A Fork, 20 To 25 Minutes. Remove Halibut From The Oven And Sprinkle With Parmesan Cheese. Place Oven Rack About 6 Inches From Broiler Heat Source And Turn On Oven'S Broiler. Place Under Preheated Broiler And Broil Until Cheese Is Lightly Browned, 2 To 3 Minutes.