1½ pounds ground beef

2 teaspoons Worcestershire sauce

1 teaspoon kosher salt

½ teaspoon ground black pepper

12 slices sourdough bread

½ cup Secret Sauce

3 medium Vidalia onions, thinly sliced

6 slices Cheddar cheese

8 tablespoons unsalted butter

Secret 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, 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. 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.