1. Slice and prepare the onions. Peel the onions and slice them into thin half-moons so they cook evenly and soften into sweet, caramelized strands.

2. Caramelize the onions. In a large skillet over medium heat, melt the butter with the olive oil. Add the sliced onions and sprinkle with the salt. Cook, stirring every few minutes, for 15–20 minutes, until the onions are deeply golden, soft, and jammy. If the pan looks dry, add a splash of water or a bit more oil. Reduce heat if they start to brown too quickly. When done, set aside.

3. Mix the secret sauce. In a small bowl, whisk together the mayonnaise, ketchup, Dijon mustard, chopped dill pickles, pickle brine, Worcestershire sauce, hot sauce, garlic powder, onion powder, and salt until smooth. Taste and adjust seasoning, adding more pickle brine for tang, ketchup for sweetness, or hot sauce for extra heat. Refrigerate until ready to use.

4. Season the ground beef. In a medium bowl, combine the ground beef, Worcestershire sauce, garlic powder, onion powder, salt, and black pepper. Gently mix with your hands or a spatula just until everything is evenly distributed, taking care not to overwork the meat.

5. Shape the patties. Divide the seasoned beef into four equal portions. Press each portion into a thin rectangle or oval that roughly matches the size of your bread slices. Place the patties on a plate or tray and refrigerate while you preheat the pan.

6. Sear the beef patties. Heat a large skillet or griddle over medium-high heat and lightly grease it with nonstick spray or neutral oil if needed. Add the patties and cook for 3–4 minutes on the first side, until well browned around the edges. Flip and cook for another 2–3 minutes, or until the centers are just cooked through. Transfer to a plate and tent loosely with foil.

7. Butter the bread slices. Lay out the bread on a work surface. Spread one side of each slice with the softened butter, making sure to go all the way to the edges so the bread toasts evenly. Flip the slices so the unbuttered side faces up.

8. Spread the secret sauce. Spoon a generous layer of secret sauce onto the unbuttered side of each slice of bread, spreading it evenly from corner to corner. Reserve a little extra sauce for serving.

9. Layer the cheese, patties, and onions. On four slices of sauced bread, place one slice of American cheese. Top each with a warm beef patty, a heaping spoonful of caramelized onions, and one slice of Swiss or cheddar cheese. Cap each stack with a second slice of bread, sauce-side down and buttered side facing out.

10. Toast the patty melts. Wipe out the skillet or griddle and set it over medium heat. Place the assembled sandwiches in the pan. Cook for 3–4 minutes per side, pressing gently with a spatula, until the bread is deep golden and crisp and the cheese is fully melted. Adjust the heat as needed so the bread browns without burning.

11. Rest and slice. Transfer the toasted patty melts to a cutting board and let them rest for 1–2 minutes so the cheese can settle slightly. Slice each sandwich in half, either straight down or on a diagonal, for easier serving.

12. Serve with extra sauce. Serve the patty melts hot with any remaining secret sauce on the side for dipping, along with your favorite sides like fries, chips, or a simple salad.