Scalloped Potatoes with Green Onions

Preheat oven to 400 degrees F. Lightly grease 13" by 9" glass or ceramic baking dish, or 2 1/2-quart gratin dish. In 5- to 6-quart saucepot, melt margarine on medium. Meanwhile, thinly slice green onions, reserving 2 tablespoons green tops. To margarine in saucepot, add sliced green onions and cook 5 minutes or until tender, stirring occasionally. Add flour and cook 1 minute, stirring constantly. With wire whisk, gradually whisk in milk. Cook on medium 6 to 7 minutes or until mixture boils and thickens, stirring constantly. Stir in potatoes, 1 teaspoon salt, and 1/2 teaspoon freshly ground black pepper. Cook 5 minutes, stirring gently; remove from heat. Transfer potato mixture to prepared baking dish, spreading evenly; sprinkle with Parmesan. Cover with foil sprayed with nonstick cooking spray, and bake 1 hour. Uncover dish; reset oven control to broil. Broil, 6 inches from heat source, 5 minutes or until Parmesan is golden. Sprinkle with reserved green onions.