1Heat oven to 375°F. Spray 2-quart casserole dish with cooking spray.
2In 1-quart saucepan, melt butter over medium heat. Stir in flour; cook 1 minute. Gradually add milk, stirring constantly with whisk. Heat to boiling; reduce heat. Simmer uncovered 5 minutes. Stir in mustard, salt and pepper. Remove from heat; stir in cheeses until melted.
3Layer half of the potatoes in casserole; pour half of the sauce over potatoes. Repeat layers once.
4Cover; bake 30 minutes. Uncover; bake 30 minutes longer or until potatoes are tender and top is golden brown. Let stand 10 minutes before serving.