In a sauce pan, cook onion in butter till tender. Stir in flour and salt. Add milk all at once. Cook and stir until thickened and bubbly. Remove from heat. Mix in 3/4 cup of cheese.

Place half the sliced potatoes in a greased 1 quart dish. Cover with half the sauce. Repeat layers. Top with remaining 1/4 cup of cheese.

Bake covered at 350 degrees for 35 minutes. Uncover and bake 30 minutes more or till potatoes are tender. Let stand 5 minutes. Serves 4. (If you don't want to use the cheese and just have plain scalloped potatoes, omit the cheese and increase the salt to 1/2 tsp.)