These scrumptuous sandwiches continue to rank among our most popular recipes! Try them out with our Oven Fries or Cheesy, Crispy Smashed Potatoes on the side.
In a large bowl, combine ground beef, Worcestershire, salt and pepper. Shape ground beef into 6 oval patties.
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 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. 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.
For carmelized onions, in a medium cast-iron skillet, melt butter over medium heat. Add onion. Cook, stirring occasionally, until onion is soft and golden brown.
To make the secret sauce, stir all sauce ingredients together in a small bowl until mixed. Store, covered, in refrigerator up to 3 days.