Butter a 6 cup souffle dish or 1 quart cassarole. Make a 4″ band of aluminum foil 2″ longer than the circumference of the dish, butter 1 side and secure it, butter side in, around the dish.
Heat butter in a saucepan over low heat until melted. Blend in flour, salt, mustard and red pepper. Cook over low heat, stirring constantly, until mixture is smooth and bubbly; remove from heat. Stir in milk. Heat to boiling, stirring constantly. Boil and stir 1 minute. Stir in cheese until melted; stir in sherry if using, let cook for about a minute. stir in bacon, remove from heat.
Beat egg whites and cream of tartar until stiff but not dry. Beat egg yolks until very thick and lemon colored, about 5 minutes; stir into cheese mixture. Stir about 3/4 of the egg whites into cheese mixture. Fold cheese mixture into remaining egg whites.
Carefully pour into prepared souffle dish. Cook uncovered until the souffle is nicely puffed and the sides look just dry around 50 – 60 minutes. Mine took 50.