#1 Cook potatoes in boiling water 20 min. or just until tender drain. Cover with cold water cool #2 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 #3 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 #4 Bake 25 to 30 min. or until heated through. Top with remaining cheese bake 5 min. or until melted.