Slice the yellow onions thickly, about half an inch, and marinate with Worcestershire sauce, olive oil, Cajun seasoning, and brown sugar for at least 15 minutes.

Heat a skillet over medium-low heat and cook the marinated onions for about 10 minutes until golden brown and tender.

Season the ground beef with Cajun seasoning, divide into four portions, and shape into patties with a small indentation in the center.

Cook patties in a hot skillet for about 2-3 minutes on one side, then flip and add cheese on top, covering until cheese melts.

Mix mayonnaise, relish, Dijon mustard, garlic powder, onion powder, smoked paprika, and cayenne pepper in a bowl for the special sauce.

Assemble by layering a slice of caramelized onion, a beef patty with cheese, some special sauce, and another slice of onion on top.