1. Preheat oven to 375°. Remove and discard root ends and dark green tops of leeks. Cut leeks into thin slices; rinse well, and drain.
2. 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.
3. Spoon potato mixture into a lightly greased 11- x 7-inch baking dish; top with cheese.
4. 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.