Patty Melts With Secret Sauce

INGREDIENTS:
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.