Preheat oven to 325°F.
Heat 3 tablespoons of butter in a saucepan over low heat until melted. Blend in flour, salt and pepper. Cook over low heat, stirring constantly, until mixture is smooth and bubbly. Gradually stir in milk, 1/2 cup at a time. Heat to boiling. Boil and stir 1 minute.
Arrange potatoes in greased 2 quart casserole in 3 layers, topping each layer with part of the chopped onions and 1/3 of the sauce. Dot the top with the remaining 1 tablespoon of butter, broken into little pieces.
Cover and bake until potatoes are tender, about 60-70 minutes. Let stand 5 to 10 minute before serving. Serves 6
Tips and Variations: Add 1/2 to 3/4 cup grated cheddar cheese to the sauce and mix to melt. Sprinkle bread crumbs on the top if the potato mixture before baking and bake as above. Easy au gratin potatoes!