Instructions
- Preheat to 400 F. Butter the casserole dish.
- Layer half of potatoes into bottom of dish. Top with onions and add remaining potatoes. Season with salt and pepper.
- In medium-sized pan, melt butter over medium heat. Mix in flour and salt. Stir constantly with whisk for 1 minute. Stir in milk.
- Cook until thickened, then stir in cheese all at once.
- Continue stirring until melted, about 30 to 60 seconds. Pour cheese over potatoes and cover with dish with aluminum foil. Bake 90 minutes.