Scalloped Potatoes

3 tablespoons butter
2 tablespoons all-purpose flour
1 1/2 teaspoons salt
1/4 teaspoon pepper
3 cups milk
6 medium-sized, pared, thinly sliced potatoes
1 cup chopped onion

Melt the butter; add flour, salt, and pepper. Add milk; stir till thick and bubbly.

Place half the sliced potatoes in a casserole dish (13x9x2-inch, usually) and cover with half the onion and half the white sauce. Repeat. Bake covered at 350°F for an hour; remove cover and bake for another half hour.