1. Wash hands with warm water and soap.
2. Wash and peel onion—dice and set aside.
3. Rinse and peel red potatoes. Slice potatoes about ⅛" thick, set aside.
4. Preheat the oven to 325 F.
5. To make the sauce. In a large saucepan, combine diced onion and 2 cups milk. Cook until onion is soft, about 3 minutes. Then, bring to a boil. Stir to prevent scalding.
6. Gradually add flour, whisking in one tablespoon at a time, until desired consistency.
7. Reduce heat to low. The sauce will become very thick, and you may need to add more milk.
8. Butter a 9"x13" baking dish. Place ⅓ of the potatoes in the bottom and season with salt and pepper. Pour ⅓ of the cream sauce over the potatoes, repeat.
9. Bake at 325 F. for 30 minutes. Check to see if you need to add additional milk. Continue baking for another 45 minutes or until potatoes are tender.
10. Remove from oven, and rest for 15 minutes before serving.

Storage: Store in the refrigerator 4 days, or freeze for longer storage.