Put all of the ingredients from the fish down to the lemon in a large kettle. Over heat, pour on boiling water or stock. Add mushrooms, cover and cook for 20 minutes. Strain the liquid into another pan and boil to reduce. Add a bit of the hot liquid to the beaten yolks, whisking constantly. Add more liquid until the yolk mixture is hot. Add dissolved arrowroot and egg mixture to the pot of broth and heat, but do not allow to boil. Place fish and shellfish on a platter and pour the sauce over them. Garnish with croutons or toasts.