Preheat oven to 350F.
Mix bisquick and mustard in pan. Add enough milk to give the mixture a paste-like consistency. Gradually stir in the rest of the milk. Boil for 1 minute, stirring constantly. Remove from heat.
Gradually stir mixture into egg yolks. Stir in tuna.
Beat egg whites and cream of tartar until stuff. Fold into tuna mixture. Pour into an ungreased baking dish, and place dish into a 1-inch deep pan of hot water. Bake for 50 to 60 minutes.