Begin by melting the butter in a skillet over medium-low heat and add the neutral oil.
Toss in the chopped onions along with the salt.
Stir occasionally as the onions caramelize—don't rush! This lovely process can take 30 minutes or more.
While the onions are working their magic, combine the mayonnaise, ketchup, rice vinegar, sweet relish, sugar, and chili flakes in a bowl to create the sauce. Set it aside.
Bake or fry the frozen fries according to the package instructions—get them crispy!
When the fries are ready, sprinkle grated cheddar or lay sliced American cheese on top. Broil for 3 to 5 minutes until the cheese is delightfully melted.
Finally, top your cheesy fries with the caramelized onions and drizzle on that delicious sauce. Enjoy every bite!