01 -
Heat butter and neutral oil in a pan over medium-low heat. Add diced onions and salt. Stir occasionally, reducing heat if onions begin to burn, and cook for at least 30 minutes until golden and caramelized.
02 -
Combine mayonnaise, ketchup, vinegar, sweet relish, sugar, and chili flakes (if using) in a bowl. Mix thoroughly and set aside.
03 -
Bake or fry the frozen fries according to the package instructions until crispy.
04 -
Place cooked fries on a baking sheet, top with grated cheddar or sliced American cheese, and broil in the oven for 3 to 5 minutes until the cheese is melted and bubbly.
05 -
Top the cheese-melted fries with caramelized onions, drizzle with the prepared sauce, and serve immediately.