Heat the oven to medium high heat.
Meanwhile, slice the lemon, chop the parsley and the scallions. Mix together with butter cubes.
With a sharp knife, make a few deep cuts on both sides of the fish. Rub salt and black pepper into the cuts and inside the fish.
Fill the fish with half of the parsley, scallions and butter mixture. Rub the rest of it into the cuts and on the skin.
Place the fish and what's left from the mixture in the middle of aluminium foil. Put slices of lemon in the cuts, close the foil like an envelope and bake in the oven for 30 minutes. Serve hot with roasted vegetables or rice.