01 -
Melt butter in a large skillet over medium-low heat and add the neutral oil. Add the diced onions and salt. Stir every few minutes, allowing onions to slowly caramelize for at least 30 minutes or until deep golden brown, reducing heat if they begin to burn.
02 -
Combine mayonnaise, ketchup, rice or white vinegar, sweet relish, granulated sugar, and chili flakes (if using) in a bowl. Mix thoroughly and set aside.
03 -
Bake or fry the frozen French fries according to the package instructions until crisp and golden.
04 -
Arrange the cooked fries on a baking tray, top evenly with American cheese slices or shredded cheddar cheese, and broil for 3 to 5 minutes until fully melted.
05 -
Transfer cheese-topped fries to a serving platter, top generously with caramelized onions, drizzle with the special sauce, and serve immediately.