Preheat oven to 350°. In a small saucepan, melt butter. Stir in the flour, salt and pepper until smooth. Gradually add milk. Bring to a boil; cook and stir until thickened and bubbly, 2 minutes. , In a greased 2-1/2-qt. baking dish, layer with a third of the potatoes, half of the hot dogs and half of the onion. Repeat layers. Top with remaining potatoes. Pour white sauce over all., Bake, covered, for 1 hour. Uncover; sprinkle with cheese. Bake until potatoes are tender, 10-15 minutes longer. If desired, garnish with green onions.