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COOK potatoes in boiling water 20 min. or just until tender; drain. Cover with cold water; cool.
MEANWHILE, melt butter in medium saucepan on medium heat. Add onions; cook and stir 2 min. Blend in flour and salt; cook 1 min. Gradually stir in milk; cook 5 min. or until thickened, stirring constantly. Add 1-1/4 cups cheddar; cook and stir 1 min. or until melted. Stir in mustard.
HEAT oven to 375°F. Peel potatoes; cut into 1/4-inch-thick slices. Layer alternately with cheese sauce in 2-qt. casserole, ending with sauce.
BAKE 25 to 30 min. or until heated through. Top with remaining cheeses; bake 5 min. or until melted.