Preheat oven to 425 degrees F (220 degrees C).

Lightly butter 1 sheet of puff pastry dough. Place another sheet nearby and butter it also. If your sheets are not as long as your fillet, extend them by adding a second sheet to each, pressing them together where they join.

Season halibut fillets with salt and pepper. Place on top of one of the sheets of puff pastry dough. Sprinkle with dill.

Put the second pastry sheet directly on top of the fillet, and press down to form a pocket all around the fillet. You should have a good bit of dough left over around the fillet. Trim the excess into the shape of a fish. Use some of the trimmings to make fins (one on top,, one bottom) and a tail. Use the blunt edge of a butter knife to make indentations in the “fins” and “tail.” Roll some of the trimmed dough into an “eye” place it.

Carefully lift the pastry onto a baking sheet, and lightly brush with butter. 

Bake in a preheated oven until pastry is puffed and golden brown, about 12 to 15 minutes.

Meanwhile, bring lemon juice to a boil in a small saucepan over high heat. Boil until almost evaporated. Reduce heat to medium, and stir in cream. Simmer until cream thickens somewhat. Stir in green onions, and season with salt and pepper. 

Serve halibut pastry with a side of sauce.