Mix all sauce ingredients in a bowl, then place in the fridge to meld the flavors.
Cook the frozen french fries according to the directions. Set aside.
To caramelize the onions, place a medium or small skillet over medium heat. Add butter and melt. Add chopped onions and cook for 5 minutes over medium heat, sautéing. Add a pinch of salt and pepper, then cook 5 more minutes, sautéing while cooking. Once the onions are browned they are done. Set aside.
Preheat the oven to broil. Cover a medium baking sheet with parchment paper or foil. Spread the cooked french fries in an even layer. Add 4 slices of American cheese over the top. Place into the oven and broil for 5 minutes, watching it closely. Once the cheese is melted they are done. Remove from oven.
Spread cooked onions over the top evenly. Spread In-N-Out Sauce over the top. Eat immediately.
