Thaw frozen halibut overnight in refrigerator, then place in baking dish and blot dry with paper towels.
Let the halibut come to room temperature while you prep other ingredients. Preheat oven to 375F/190C.
Slice 3 T green onions, then measure 2 T and finely chop them. Reserve the other 1 T sliced onions to garnish the cooked dish.
In a small bowl, stir together the sour cream, garlic powder, and dill weed. Stir in the finely grated Parmesan cheese and green onions.
When fish has come to room temperature, use a rubber scraper to spread an even coating of this topping over each piece of the fish, using all the topping mixture.
Bake fish until the internal temperature reaches 145F/62C, or until fish pieces feel firm but not hard to the touch. (Use a fork to press down to test.) I baked my thick pieces of halibut for 24 minutes, but other types of fish or thinner pieces will cook more quickly.
Serve hot, sprinkled with remaining sliced green onions.
