– 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 (recipe below)
– 3 medium Vidalia onions, thinly sliced
– 6 slices Cheddar cheese
– 8 tablespoons unsalted butter
– ¼ cup Dijon mustard
– ¼ cup mayonnaise
– 1 tablespoon barbecue sauce
– ½ teaspoon hot sauce
Secret Sauce Instructions:
In a small bowl, stir together mustard, mayonnaise, barbecue sauce, and hot sauce. Store, covered, in the refrigerator for up to 3 days.
1. In a large bowl, combine ground beef, Worcestershire sauce, salt, and pepper. Shape the ground beef into 6 oval patties.
2. In a medium cast-iron skillet, melt 2 tablespoons of butter over medium heat. Add the thinly sliced onions. Cook, stirring occasionally, until the onions are soft and golden brown, approximately 35 minutes.
3. In a large cast-iron skillet, cook the patties over medium-high heat until they are browned and cooked through, approximately 2 minutes per side. Remove the skillet from heat and take out the patties; wipe out the skillet.
4. Layer 1 slice of sourdough bread with 1 tablespoon of Secret Sauce, 2 to 3 tablespoons of caramelized onions, 1 slice of Cheddar cheese, 1 patty, and another 1 tablespoon of Secret Sauce. Top with another slice of sourdough bread. Repeat with the remaining bread, Secret Sauce, caramelized onions, cheese, and patties.
5. Heat the skillet over medium-high heat. Melt 2 tablespoons of butter in the skillet. Working in batches, cook the sandwiches, flipping once, until they are golden brown and heated through, approximately 3 minutes per side. Add the remaining butter to the skillet as needed.