01 - 
                Whisk together mayonnaise, Dijon mustard, barbecue sauce, and hot sauce in a small bowl until smooth and creamy with no lumps.
              
              
              
                02 - 
                Combine ground beef, Worcestershire sauce, salt, and black pepper gently in a large bowl using your hands until evenly mixed without overworking.
              
              
              
                03 - 
                Divide beef mixture into six equal portions and shape into oval patties slightly wider and thin enough to fit bread slices, pressing lightly to hold shape.
              
              
              
                04 - 
                Melt butter in a large skillet over medium heat. Add sliced onions and cook slowly, stirring often for 20–25 minutes until onions are soft, golden, and fragrant.
              
              
              
                05 - 
                Heat a separate skillet over medium-high heat. Cook patties for approximately 2 minutes per side until browned but still juicy. Remove and wipe skillet clean.
              
              
              
                06 - 
                Spread a thin layer of softened butter on one side of each marble rye slice, preferably while bread is cool to prevent tearing.
              
              
              
                07 - 
                Place half the bread slices buttered side down in a skillet. Spread one tablespoon of secret sauce on each. Add about three tablespoons caramelized onions, a slice of cheddar, a cooked beef patty, a slice of Swiss cheese, more caramelized onions, a drizzle of secret sauce, and cap with remaining bread slices, buttered side out.
              
              
              
                08 - 
                Cook sandwiches over medium heat, pressing gently with a spatula to promote melting. When the bottom is golden, flip and repeat until cheese is melted and bread is crisp. Let rest one minute before slicing.