Heat oven to 350° F. Butter 4 cup soufflé dish or 1 quart casserole. Make a 4 inch band of triple thickness aluminum foil 2 inches longer than circumference of dish; butter 1 side. Secure foil band, buttered side in, around dish.
Heat butter in saucepan over low heat until melted. Blend in flour, salt, mustard and red pepper. Cook over slow heat, stirring constantly, until mixture is smoothe and bubble; remove from heat. Stir in milk. Heat to boiling, stirring constantly. Boil and stir 1 minute. Stir in cheese until melted; 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 1/4 of the egg whites into cheese mixture. Fold cheese mixture into remaining egg whites.
Carefully pour into soufflé dish. cook uncovered until knife inserted
halfway between center and edge comes out lean, 50 to 6 mimutes. Serve
immediatly. Carefully remove foil band and divide soufflé into
sections with 2 forks.