In a greased 2-1/2-qt. baking dish, layer potatoes and onion.
In a saucepan, melt the butter; stir in flour until smooth. Gradually add broth, mayonnaise, salt and pepper; cook and stir for 2 minutes or until thick and bubbly. Pour over potatoes. Sprinkle with paprika.
Cover and bake at 325 degrees for 2 hours or until tender.