Heat oven to 350° F. Grease 2 qt. casserole with shortening or cooking spray.
In a saucepan, melt 3 Tbsp. of the butter over medium heat. Cook onion in butter about 2 minutes, stirring occasionally, until tender. Stir in flour, salt, and pepper. Cook, stirring constantly, until smooth. Remove from heat. Stir in milk. Heat to boiling, stirring constantly. Boil and stir 1 minute.
Spread potatoes in casserole. Pour sauce over potatoes. Cut remaining 1 Tbsp. butter into small pieces; sprinkle over potatoes.
Cover; bake 30 minutes. Uncover; bake 1 hour or until potatoes are tender. Let stand 5-10 minute before serving.
Food for Thought
Think about Ham Base. Blending a little RBC Ham Base into the milk will give these potatoes a great flavor. Think 1 tsp. of the base to each cup of milk.