The recipe yield is:
Separate egg yolks from whites, putting whites into a small deep bowl and yolks into a medium sized bowl. Beat yolks slightly. Grease and flour a 1 quart casserole or other baking dish. Melt the butter, remove from heat and stir in flour and mustard. When smooth stir in milk. Cook, stirring constantly until mixture comes to a full boil. Beat yolks while adding sauce in a thin stream. Stir in cheeses. Heat oven to 375øF. Beat egg whites and salt with an electric mixed until peaks hold when beater is lifted. Stir in a heaping tablespoon of the beaten whites into the yolk mixture, then carefully fold in the remaining whites. Pour into prepared baking dish and smooth the surface. Bake at 375øF for 10 to 15 minutes for a shallow dish and 20 to 25 minutes for a casserole, until souffle‚ is puffed and browned. Serve immediately. From: Syd's Cookbook.