Heat oven to 350°F. Grease 2-quart baking dish.
In a saucepan, melt 3 tablespoons of butter over medium heat. Add onion and cook for 2 minutes. Stir in flour, salt and pepper. Cook, stirring constantly, until roux is created; remove from heat.
Stir in milk. Place back on the burner and heat to boiling, stirring constantly. Let boil, while stirring for 1 minute, until mixture becomes a thick sauce.
Spread Reser’s Sliced Potatoes in baking dish. Pour sauce over potatoes. Cut remaining 1 tablespoon butter into small pieces and sprinkle over potatoes.
Cover; bake 30 minutes. Uncover; bake 1 hour to 1 hour 10 minutes longer or until potatoes are tender. Let stand 5 to 10 minutes before serving.
Garnish the potatoes with chopped chives and serve the scalloped potatoes family style on the side of delicious entree like Fried Chicken.