Cook fries according to package directions.

Preheat a large frying pan over medium-low heat, add the butter, and let it melt.

Peel and chop the onion into small pieces and add it to the frying pan.

Add the brown sugar and salt to the onion and let it cook slowly.

Add water if necessary.

In a small bowl, add the mayonnaise, ketchup, sweet pickle relish, sugar, and vinegar.

Whisk to combine.

On a serving platter, start by placing the hot fries as the bottom layer.

Quickly lay on the slices of cheese in an even layer and allow the hot fries to melt the cheese. Cover the cheese with a layer of onions and add the sauce as the top layer.

Enjoy!