How to Make It
Preheat oven to 375°. Remove and discard root ends and dark green tops of leeks. Cut leeks into thin slices; rinse well, and drain.
Cook leeks in hot oil in a large skillet over medium heat, stirring often, 5 minutes or until lightly golden. Stir in potatoes and next 3 ingredients; cover and cook, stirring occasionally, 20 minutes or until potatoes are slightly tender. Drain potato mixture. Add Alfredo sauce and salt, gently stirring just until coated.
Spoon potato mixture into a lightly greased 11- x 7-inch baking dish; top with cheese.
Bake, covered, at 375° for 20 minutes; uncover and bake 15 more minutes or until potatoes are tender and cheese is lightly browned.
To test gratin for doneness, pierce potatoes with a long wooden pick. The gratin is ready when the pick slides easily through the potatoes.