01 -
In a small bowl, mix together the mayonnaise, Dijon mustard, barbecue sauce, and hot sauce until smooth.
02 -
In a large mixing bowl, combine the ground beef, Worcestershire sauce, salt, and black pepper. Mix gently just until incorporated.
03 -
Divide the beef mixture evenly and form into 6 oval-shaped patties of similar thickness.
04 -
Melt the butter in a skillet over medium heat. Add the sliced onions and cook, stirring occasionally, until the onions are soft and golden brown.
05 -
Heat a separate skillet over medium-high heat. Cook the beef patties for 2 minutes per side, until browned but still juicy. Remove patties and wipe the skillet clean.
06 -
Spread a thin layer of butter on the outer sides of each slice of bread.
07 -
Place buttered bread slices, buttered side down, on the skillet. Top with 1 tablespoon of secret sauce, 3 tablespoons of caramelized onions, a slice of cheese, a beef patty, a second slice of cheese, more onions and secret sauce, then top with another bread slice, buttered side up.
08 -
Cook sandwiches on both sides over medium heat until the bread is golden brown and the cheese is melted. Repeat with remaining sandwiches.