1 1/2 pounds ground beef
2 teaspoons Worcestershire sauce
1 teaspoon kosher salt
1/2 teaspoon ground black pepper
12 slices sourdough bread
1/2 cup Secret Sauce
3 medium Vidalia onions, thinly sliced
6 slices Cheddar cheese
8 tablespoons unsalted butter
1/4 cup Dijon mustard
1/4 cup mayonnaise
1 tablespoon barbecue sauce
1/2 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.