Mix together butter, garlic, lemon zest and juice.
Season halibut with salt and pepper. Divide half of the butter mixture between the 2 steaks and spread it over one side.
Sprinkle flour into large oven roasting bag (about 12 by 18 inch size) and shake bag to dust inside with flour. Place bag in a 10 by 15-inch baking pan, about 2 inches deep. Place halibut steaks side by side, buttered side down, in the bag.
Arrange shrimp on top of fish, then top with clams and dot with remaining butter.
Seal bag with twist tie according to manufacturer's instructions, and cut 4 to 6 small slits in top.
Place in preheated oven and bake 30 to 40 minutes or until clams have opened.
Slash top of bag just enough to remove food without losing juices. With slotted spoon, lift out clams and set aside. With slotted spatula, lift out fish and shrimp and place on a warm serving platter and surround with clams; keep warm.
Pour cooking liquid into a wide pan, add cream and cook over high heat until liquid is reduced to about 3/4 cup.
Garnish fish with parsley and pass sauce to pour over each serving.