To make sauce, cook green onion in butter until crisp-tender. Stir in flour, 1 1/4 teaspoon salt, and 1/8 teaspoon pepper. Add milk. Cook and stir until thickened and bubbly. Stir in sour cream until smooth.
Place half of the sliced potatoes in a grease 2 quart casserole. Cover with half of the sauce. Repeat layers.
Bake, covered, in a 350 degree oven for 45 minutes, stirring once. Uncover, bake about 30 minutes more or until potatoes are done. Makes 4-6 servings.