Patty Melts con Salsa Segreta 

Prepare the Patties

In a bowl, combine ground beef, Worcestershire sauce, garlic powder, salt, and pepper. Form into 4 thin patties.
Cook the Onions

Melt butter in a skillet over medium heat. Add onions and cook until caramelized, about 10–15 minutes. Set aside.
Cook the Patties

Cook patties in the same skillet over medium heat for 3–4 minutes per side.
Assemble the Sandwiches

Spread secret sauce on one side of each bread slice. Layer with a patty, a slice of Swiss cheese, caramelized onions, and a slice of cheddar cheese. Top with another slice of bread.
Grill the Patty Melts

Grill sandwiches in a skillet over medium heat, pressing lightly, until the bread is golden and the cheese is melted.
Serve

Slice in half and serve warm.
Buon appetito!