1-½ lbs. ground beef
2 Tsps. Worcestershire sauce
1 Tsp. kosher salt
½ Tsp. ground black pepper
12 Slices sourdough bread
½ C. Secret Sauce
3 Med. Vidalia onions, thinly sliced
6 Slices Cheddar cheese
8 Tbsps. unsalted butter
¼ C. Dijon mustard
¼ C. mayonnaise
1 Tbsp. barbecue sauce
½ Tsp. hot sauce
In a small bowl, stir together mustard, mayonnaise, barbecue sauce, and hot sauce. Store, covered, in refrigerator up to 3 days.
HOW TO MAKE:
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 TBSPS. butter over medium heat. Add onion. Cook, stirring occasionally until onion is soft and golden brown, approximately 35 mins.
In a large cast-iron skillet, cook patties over medium-high heat until browned and cooked through, approximately 2 mins per side. Remove skillet from heat. Remove patties from skillet; wipe out skillet.
Layer 1 bread slice with 1 tablespoon Secret Sauce, 2-3 Tbsps. caramelized onions, 1 slice cheese, 1 patty, and another 1 Tbsp. 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 Tbsps. butter in skillet. Working in batches, cook sandwiches, flipping once, until golden brown and heated through, approximately 3 mins per side. Add remaining butter to skillet as needed.