Patty Melts with Secret Sauce

Patty Melts with Secret Sauce 1½ pounds ground beef 2 teaspoons Worcestershire sauce 1 teaspoon kosher salt ½ teaspoon ground black pepper 12 slices sourdough bread or my favorite-seeded rye bread ½ cup Secret Sauce 3 medium Vidalia onions, thinly sliced 6 slices Cheddar cheese 8 tablespoons unsalted butter Sauce ¼ cup Dijon mustard ¼ cup mayonnaise 1 tablespoon barbecue sauce ½ teaspoon hot sauce In a small bowl, stir together mustard, mayonnaise, barbecue sauce, and hot sauce. Store, covered, in refrigerator up to 3 days. 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. 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. 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.