In a small bowl, mix mayonnaise, ketchup, Dijon mustard, Worcestershire sauce, and pickle relish until well combined. Set aside.

Cook the Onions

Heat a large skillet over medium heat. Add a drizzle of oil and the sliced onions. Cook, stirring occasionally, until the onions are golden and caramelized, about 15–20 minutes. Remove from the skillet and set aside.

Make the Beef Patties

Season the ground beef with garlic powder, onion powder, salt, and pepper. Divide the beef into 2 or 4 equal portions and shape into thin patties slightly larger than the bread slices.

In the same skillet, cook the patties over medium-high heat for 3–4 minutes per side, or until browned and cooked through. Remove and set aside.

Assemble the Sandwiches

Spread secret sauce on one side of each slice of bread.

Layer a slice of Swiss cheese, a beef patty, caramelized onions, and another slice of Swiss cheese on half of the bread slices. Top with the remaining bread, sauce side down.

Grill the Sandwiches

Butter the outside of each sandwich. Heat the skillet or griddle over medium heat. Cook the sandwiches until the bread is golden brown and the cheese is melted, about 3–4 minutes per side.

Serve

Slice each patty melt in half and serve hot with extra secret sauce on the side.