In a skillet, combine butter and neutral oil over medium-low heat. Add the diced onions and sprinkle with a pinch of salt. Stir often until the onions are golden and caramelized, about 30 minutes.

In a mixing bowl, whisk together mayonnaise, ketchup, rice vinegar, sweet relish, sugar, and chili flakes. Set aside to let the flavors meld.

Bake or fry the frozen fries according to the package instructions until crispy and golden brown, usually around 20-25 minutes at 425°F.

Once fries are cooked and piping hot, heap on the grated cheddar or sliced American cheese. Broil for 3-5 minutes until the cheese is melted and bubbly.

Layer the sweet caramelized onions on top of the melted cheese, then drizzle the special sauce over all the fries. Serve immediately.