Mix all burger sauce ingredients together until combined. Cover and let sit in the fridge while you make the rest of the ingredients. The longer the sauce sits, the deeper the flavor gets.
Place the French fries in a preheated air fryer or oven and cook according to package instructions. Once done, place them in a cast iron skillet or baking pan. Set aside.
In a medium skillet, add butter and diced sweet onion. Place over medium heat. Cook until onion turns soft and starts to char a little, about 10 minutes. Stir ocassionally.
Set oven to broil. Sprinkle diced onion over the cooked fries in the cast iron. Top with American cheese. Place under broiler until the cheese is very very melty but not burning. Remove from oven and drizzle with half of the burger sauce. Serve the rest on the side.