To make the stuffing: Melt butter or margarine in a large saucepan over a medium-low heat. Saute onion, celery and parsley until all of the vegetables are just tender. Mix in crabmeat, prawns and vermouth. Season with salt, pepper and Tabasco sauce; cook until prawns are finished cooking (they will be pink). Mix in bread crumbs, a little at a time. When the mixture holds together well, stop adding bread crumbs. Taste and add more seasoning (salt, pepper and Tabasco sauce) if necessary. Set this mixture aside to let it cool.
Spray a baking tray with non-stick cooking spray.
Roll 1 sheet of puff pastry onto a flat surface. The puff pastry once rolled should be about 5mm thick and large enough for you to lay the fish on top of it and still have puff pastry on the sides. Lay one of the fish fillets on top of the puff pastry. Spread the stuffing mixture evenly over the fish fillet. Place the remaining fillet over the stuffing. Trim the pastry around the filets in roughly the shape of a fish. Save the trimmings.
Roll second sheet of puff pastry out to about 5mm thick. Drape second sheet over stuffed fillets, making sure that there is enough of the top sheet to tuck under the bottom sheet of puff pastry. Trim the top sheet of pastry about 1cm larger than the bottom sheet. Brush underside of top pastry sheet with water and tuck under bottom sheet of puff pastry pressing lightly to totally encase the fish and stuffing packet. Place the sealed packet on the prepared baking tray, and let it cool for 10 to 15 minutes.
While packet is chilling, roll out pastry scraps. From the scraps cut out fins, an eye and 'lips'. Attach cut-outs to chilled packet with a little water. Use an inverted teaspoon to make indentations in puff pastry to resemble fish scales but do not puncture pastry. Chill entire packet.
While the packet is chilling, preheat the oven to 220 degrees C.
Remove the fish from the refrigerator and brush the packet with the egg yolks. Measure the thickness of the packet at its thickest part. Bake for 15 minutes, then reduce the temperature to 180 degrees C and bake the fish for 5 extra minutes per centimetre of measured thickness. Test for doneness by inserting a thermometer into the packet, when the temperature reaches 70 degrees C the fish is finished cooking.