To make the sauce, place the ingredients in a bowl and stir to combine. Cover and refrigerate for at least 4 hours before serving for the best flavor. This can be made up to several days in advance.
While the fries are baking make the caramelized onions: Heat the butter in a skillet over medium heat, add the onions, salt and sugar, and cook the onions until they're nicely browned.
To serve place the French fries on a plate, top with sliced American cheese or shredded Cheddar cheese, top with caramelized onions and spoon over some sauce. Serve immediately.Note: If using shredded Cheddar, plate the fries, sprinkle the cheese over them and microwave briefly until melted, then top with onions and sauce.