Cook potatoes in boiling water 20 min. or just until tender; drain. Cover with cold water; cool
Melt butter in a medium saucepan over medium heat. Add onions; cook 2 min, stirring. Stir in flour and salt; cook 1 min. Gradually whisk in milk; cook 5 min until thick. Add 1¼ cups cheddar and mustard; stir until melted and smooth
Heat oven to 375°F. Peel potatoes; cut into 1/4-inch-thick slices. Layer alternately with cheese sauce in 2-qt. casserole sprayed with cooking spray, ending with sauce
Bake 25 to 30 min. or until heated through. Top with remaining cheese; bake 5 min. or until melted
