Mix the beef and turkey together with all of the seasonings, egg, and breadcrumbs together until smooth. Form meat mixture into patties and grill on a barbecue grill until cooked through. For the last minute of cooking, place a thick slice of pepper jack cheese to your burger. Let cook until melted.
Mix the mayonnaise ingredients together until creamy and well mixed.
Assemble the burgers together with a generous helping of the Cajun Mayo, the burger, lettuce, tomatoes, and onions. Serve while warm. Makes 6 large burgers.