Combine all sauce ingredients in a bowl, then set in the fridge to use when the fries are done.
Follow the instructions on the bag to cook those frozen french fries to perfection. Once done, set them aside. (If using fresh potatoes, slice and fry at 350° until golden.
Heat a medium or small skillet over medium heat. Add butter and allow it to melt. Add chopped onions and cook for about 5 minutes, stirring occasionally. Season with a pinch of salt and pepper. Continue sautéing for another 5 minutes until onions are golden brown and caramelized. Once caramelized, remove from heat and set aside.
Preheat the oven to broil. Line a medium baking sheet with parchment paper or foil. Spread the cooked french fries in an even layer on the sheet. Place 4 slices of American cheese over the fries. Broil in the oven for about 5 minutes, keeping a close eye to ensure the cheese is melted but not burnt.
Remove from oven when cheese is melted. Sprinkle the caramelized onions evenly over the fries. Dollop the In-N-Out Sauce over the top. Serve and enjoy!