1Heat oven to 350°F. Butter 1-quart soufflé dish or casserole. Make a 4-inch band of triple-thickness aluminum foil 2 inches longer than circumference of dish. Butter one side. Secure foil band, buttered side in, around top edge of dish.
2Melt butter in 2-quart saucepan over medium heat. Stir in flour, salt, mustard and red pepper. Cook over medium heat, stirring constantly, until smooth and bubbly; remove from heat. Stir in milk. Heat to boiling, stirring constantly. Boil and stir 1 minute. Stir in cheese until melted; remove from heat.
3Beat egg whites and cream of tartar in medium bowl with electric mixer on high speed until stiff but not dry; set aside. Beat egg yolks in small bowl on high speed about 3 minutes or until very thick and lemon colored; 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.
4Bake 50 to 60 minutes or until knife inserted halfway between center and edge comes out clean. Carefully remove foil band and quickly divide soufflé into sections with 2 forks. Serve immediately.