Ultimate Patty Melt with Secret Sauce

In a small bowl, mix together mayonnaise, ketchup, relish, Worcestershire sauce, garlic powder, and onion powder.
Season with salt and pepper to taste. Set aside.
Caramelize the Onions:

In a skillet over medium heat, melt 1 tbsp of butter.
Add the sliced onions, season with salt and pepper, and cook for 10-15 minutes, stirring occasionally, until golden and caramelized.
Remove from the skillet and set aside.
Cook the Beef Patties:

Divide the ground beef into 4 equal portions and shape into patties slightly larger than the bread slices.
Season both sides with salt and pepper.
In the same skillet over medium heat, cook the patties for about 4 minutes per side (or to your desired doneness).
Remove from heat and set aside.
Assemble the Patty Melts:

Butter one side of each slice of bread.
On the unbuttered side of 4 slices, spread a generous layer of the secret sauce.
Place 1 slice of Swiss cheese, followed by a cooked patty, caramelized onions, another slice of cheese, and then top with a second slice of bread (buttered side facing out).
Grill the Sandwiches:

Heat a clean skillet or griddle over medium heat.
Cook each sandwich for 3-4 minutes per side, pressing gently with a spatula, until golden brown and the cheese is melted.
Serve & Enjoy:

Slice in half and serve warm with fries, pickles, or a side salad.
Serving and Storage Tips