1. Preheat oven to 400 degrees Fahrenheit (200 degrees Celsius).
2. Place the French-fried onions into a food processor and process until finely ground. Transfer to a bowl and add grated Parmesan cheese, mixing well until combined.
3. Wash and dry potatoes well. Slice in half and place in a large bowl. Add two tablespoons oil, spices, and horseradish sauce. Mix with a large spoon until potatoes are fully coated.
4. Prepare two 9×13-inch (23×33-centimeter) aluminum pans. Drizzle three tablespoons oil into each pan. Tilt the pan to spread oil all over the base. Use a spoon to sprinkle the Parmesan mixture over the oil in each pan. Spread as evenly as you can. Once sprinkled, don’t try to spread it more. Place potatoes cut side down into the Parmesan/oil mixture, pressing firmly.
5. Sprinkle potatoes with some additional salt. Bake uncovered for 35–40 minutes or until potatoes are soft and the Parmesan crust is deep golden.
6. Remove pans from oven and allow to rest for five to seven minutes. This step is important, as it helps the cheese crust stick to the potatoes. Then use a knife to cut between every few potatoes, cutting through the Parmesan crust that connects the potatoes to each other. Using a spatula, scrape up every three or so potatoes and flip them over so the cheese side is facing upwards, then slide onto a serving dish.