Preheat oven to 350 degrees Fahrenheit (175 degrees Celsius). Spray a 9 x 13-inch (23 x 33-centimeter) glass or ceramic baking dish with cooking spray.

Heat oil in a large pot over medium-high heat. Add the onions and sauté until translucent. Add the potato starch, mayonnaise, salt, and chicken or vegetable broth. Stir until smooth.

Cook until the sauce thickens. Add horseradish and mix well.

With a ladle, place a layer of the sauce into the bottom of the prepared pan. Cover with a layer of overlapping potatoes. Top with a layer of sauce. Repeat, alternating layers of potatoes and sauce. There should be a total of three sauce layers and two potato layers, with sauce on top.

Sprinkle the top with pepper and paprika. Bake uncovered for one and a half hours or until golden.