Preheat oven to 350 degrees. Cut halibut into 6 serving size blocks. Place in a single layer in a shallow baking dish.
Cream butter with onions, salt, marjoram, pepper and parsley, stir in lemon juice. Spread over halibut; cover. Bake 35 minutes or until fish flakes easily with a fork. Serve from baking dish. Garnish with parsley sprigs or lemon wedges.