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° for 2 hours or until tender.