Mix potatoe chips with 1/2 cup of the cheese and set aside. Combine
the mashed potatoes, egg yolks, and mustard. Stir in tuna and
remaining 1/2 cup of cheese.
Beat egg whites until stiff, Fold into the potatoe-tuna mixture. Turn
into a greased 2 qt baking dish. Sprinkle evenly with potatoe chip
mixture.