1Heat oven to 350°F. Grease 2-quart casserole with shortening or cooking spray.
2In 2-quart saucepan, melt 3 tablespoons of the butter over medium heat. Cook onion in butter about 2 minutes, stirring occasionally, until tender. Stir in flour, salt and pepper. Cook, stirring constantly, until smooth and bubbly; remove from heat. Stir in milk. Heat to boiling, stirring constantly. Boil and stir 1 minute.
3Spread potatoes in casserole. Pour sauce over potatoes. Cut remaining 1 tablespoon butter into small pieces; sprinkle over potatoes.
4Cover; bake 30 minutes. Uncover; bake 1 hour to 1 hour 10 minutes longer or until potatoes are tender. Let stand 5 to 10 minutes before serving (sauce thickens as it stands).