Preheat the oven to 475 degrees F (245 degrees C). Grease a 9x13 inch baking dish. Season halibut with salt and ground black pepper. Place halibut in the greased baking dish. Combine green onions, mayonnaise, sour cream, and dill in a bowl. Spread green onion mixture evenly over each halibut piece. Bake in the preheated oven until fish is opaque and flakes easily with a fork, about 20 minutes. Remove from the oven and let stand for 5 minutes before serving.