In a small bowl, mix mayonnaise, ketchup, relish, Worcestershire sauce, garlic powder, onion powder, salt, and pepper. Adjust seasoning to taste and set aside.
Caramelize the Onions:
In a skillet over medium heat, melt 1 tbsp butter. Add thinly sliced onions, season with salt and pepper, and cook until caramelized (about 10–15 minutes). Transfer to a plate.
Cook the Patties:
Form the ground beef into 4 patties, slightly larger than the bread slices. Season with salt and pepper. In the same skillet, cook each patty about 4 minutes per side for medium doneness. Remove and set aside.
Assemble the Sandwiches:
Butter one side of each bread slice. On the non-buttered side of 4 slices, spread secret sauce, then layer a slice of cheese, a patty, caramelized onions, another slice of cheese, and top with the remaining bread slice (buttered side up).
Grill the Patty Melts:
In a clean skillet over medium heat, grill each sandwich until golden brown and cheese is melted (about 3–4 minutes per side).
Serving and Storage Tips
