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
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
Secret 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.
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.