Preheat oven to 350 degrees. Melt 2 tablespoons butter in large saucepan. Stir in flour, salt, onion salt, pepper and paprika. Gradually stir in milk and cook over low heat, stirring constantly until sauce boils and thickens. Add potatoes. Heat until sauce boils again. Turn potatoes and sauce into 22 quart baking dish. Dot with 1 tablespoon butter. Bake uncovered for 40 minutes.