Heat oven to 350°F (175°C). Place aluminum foil on a large baking sheet. Place fish on foil.
Arrange lemon slices and green onion inside cavity of fish. Wrap foil sheets around fish and seal carefully.
Bake fish for 30 minutes. Remove from oven and carefully turn fish over and bake for another 10 minutes.
Check to see if fish is done by carefully unwrapping foil and inserting a knife in the thickest part. If fish is still raw in the center, return to oven for 10 minutes.
When fish is done, carefully unfold foil, exposing the top side. Peel skin off from the top side and then, holding foil, flip fish over to the other side, on to a sheet of plastic wrap. Remove skin from this side.
Using the plastic wrap, roll fish onto an attractive serving dish and garnish with more lemon slices and parsley