In a large bowl, combine ground beef, Worcestershire, salt, and pepper. Shape ground beef into 6 oval patties.
In a medium cast-iron skillet, melt 2 tablespoons butter over medium heat. Add onion. Cook, stirring occasionally, until onion is soft and golden brown, approximately 35 minutes.
In a large cast-iron skillet, cook patties over medium-high heat until browned and cooked through, approximately 2 minutes per side. Remove skillet from heat. Remove patties from skillet; wipe out skillet.
Layer 1 bread slice with 1 tablespoon Secret Sauce, 2 to 3 tablespoons caramelized onions, 1 slice cheese, 1 patty, and another 1 tablespoon Secret Sauce. Top with another bread slice. Repeat with remaining bread, Secret Sauce, caramelized onions, cheese, and patties.
Heat skillet over medium-high heat and melt 2 tablespoons butter in skillet. Working in batches, cook sandwiches, flipping once, until golden brown and heated through, approximately 3 minutes per side. Add remaining butter to skillet as needed.
Secret Sauce
Ingredients
1/4 Cup Dijon mustard
1/4 Cup mayonnaise
1 Tablespoon barbecue sauce
1/2 Teaspoon hot sauce
Preparation
In a small bowl, stir together mustard, mayonnaise, barbecue sauce, and hot sauce, store, covered, in refrigerator up to 3 days.