1Heat oven to 350°F. Grease 13x9-inch (3-quart) glass baking dish with shortening or cooking spray.
2In 2-quart saucepan, melt butter over medium heat. Stir in flour with wire whisk until smooth. Gradually stir in milk. Heat to boiling. Reduce heat to low; cook about 5 minutes, stirring frequently, until sauce is slightly thickened. Stir in mustard and salt.
3Place half of the potatoes in baking dish; top with half of the sauce and half of each of the cheeses. Repeat layers.
4Bake 50 to 55 minutes or until potatoes are tender and top is golden brown. Let stand 5 to 10 minutes before serving.