1/2 cup butter or margarine
1/2 cup Gold Medal® all-purpose flour
3 cups milk
1 tablespoon Dijon mustard
1/2 teaspoon salt
10 cups thinly sliced (about 1/8 inch) Yukon gold potatoes (about 3 lb)
1 1/2 cups shredded Gruyère cheese (6 oz)
1 1/2 cups shredded Cheddar cheese (6 oz)
1 Heat oven to 350°F. Grease 13×9-inch (3-quart) glass baking dish with shortening or cooking spray.
2 In 2-quart saucepan, melt butter over medium heat. Stir in flour with wire whisk until smooth. Gradually stir in milk. Heat to boiling. Reduce heat to low; cook about 5 minutes, stirring frequently, until sauce is slightly thickened. Stir in mustard and salt.
3 Place half of the potatoes in baking dish; top with half of the sauce and half of each of the cheeses. Repeat layers.
4 Bake 50 to 55 minutes or until potatoes are tender and top is golden brown. Let stand 5 to 10 minutes before serving.