1. Preheat oven to 350 and grease a 2 quart casserole dish with nonstick cooking spray. Set aside. 2. In a large saucepan, add butter and cook on low heat until melted. 3. Stir in flour, salt and pepper. 4. Add milk, a little at a time, and mix, continuing to cook over medium heat. 5. Stir frequently until mixture begins to boil. 6. Continue to stir and boil for 2 minutes or until thick. 7. Remove from heat. Stir in onion and cheese and mix well. 8. Add potatoes and stir well to coat. 9. Pour into prepared pan. Cover tightly with foil and bake for 60 minutes. 10. Remove foil and discard. Return to oven for an additional 20-30 minutes or until top begins to brown..