Combine the potato, flaked tuna, tuna juice and the egg yolks.
Stir in all remaining ingredients except egg whites and cheese.
Beat whites until soft peaks form, then fold into tuna mixture.
Spread in a greased 9 X 6-inch oblong bake dish; sprinkle with cheese.
Oven-poach in a pan of hot water at 375F for 30 minutes, or until set and nicely puffed.
Pass hot mushroom sauce.