Saute onions and mushrooms in butter. Add flour, pepper and paprika and blend. Remove from heat, add milk, broth and worcestershire sauce. Boil for 1 minute. Add egg yolk slowly, then sherry. Mix in the halibut and pour into 6 individual dishes and sprinkle with parmesan cheese and bread crumbs, and a dash of paprika. Bake at 450 degrees F. for 5-10 minutes, or till heated through. Garnish with fresh parsley. Source: Alaska Seafood Cookbook Reprinted by permission from the Alaska Seafood Marketing Institute