Cook fish in salted, boiling water with onion until tender, (1 teaspoon salt to 1 qt. boiling water). Drain and shred. To one cup of the hot White Sauce, add the shredded fish and then the stiffly beaten whites. Place in center of hot platter and set in oven with oven door open. Take the boiled shrimp, remove the shell and break in small pieces. Add the wine to the remaining cup of White Sauce, and pour the hot sauce gradually on the beaten yolks. Mix the beaten yolks with a little cold water. Add the shrimp, heat thoroughly and place as border around the halibut and serve at once.