Potato Au Gratin

What You Need :
                               * 2 1/2 lb.baking potatoes ( about 6 ).
                               * 1/4 Cup (1/2 stick) butter.
                               * 1 small onion, chopped.
                               * 1/4 cup flour.
                               * 1 tsp salt.
                              * 2 1/2 cups milk.
                              * 1 3/4 cups Kraft Shredded Sharp Cheddar Cheese, divided.
                              * 1/2 tsp. Grey Poupon Dijon Mustard.
                              * 1/4 cup Kraft Grated Parmesan Cheese.

Make It :
             Cook potatoes in boiling water 20 minutes. or just until tender, drain.Cover with cold
                             water, cool.
              MeanWhile, melt butter in medium saucepan on medium heat. Add onion, cook and
                              stir 2 minutes. Blend in flour and salt; cook 1 minute. Gradually sitr in milk;
                             cook 5 minutes, or until thickened, strirring constantly. Add 1- 1/4 cups
                             Cheddar; cook and stir 1 minute, or until melted. Stir in mustard.
            HEAT oven to 375 degree 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 minutes. or until heated through. Top with remaining cheeses ;
                             bake 5 minutes. or until melted.