Prepare the Secret Sauce: In a small mixing bowl, combine the mayonnaise, ketchup, Dijon mustard, and sweet pickle relish. Stir until smooth and well blended. Set this flavorful sauce aside for later use.
Cook the Onions: Heat the olive oil in a medium skillet over medium heat. Add the sliced onions along with a pinch of salt. Sauté the onions, stirring occasionally, until they are caramelized and golden brown, which should take about 10-15 minutes. Once done, transfer the onions to a plate and set them aside.
Form the Patties: In a mixing bowl, season the ground beef with a good sprinkle of salt and black pepper. Divide the beef into four equal portions and shape each portion into a patty, making them slightly larger than the bread slices since they will shrink while cooking.
Cook the Patties: Using the same skillet that you cooked the onions in, add a little more olive oil if needed. Place the beef patties in the skillet over medium-high heat. Cook for 4-5 minutes per side, or until they reach your desired level of doneness. In the final minute of cooking, place a slice of Swiss cheese on each patty to allow it to melt perfectly.
Assemble the Melt: While the cheese is melting on the patties, butter one side of each slice of rye bread. On the unbuttered side of two slices, generously spread a layer of the prepared secret sauce. Top with the melty patty, add a generous portion of the caramelized onions, and cover with another slice of bread, ensuring the buttered side is facing out.
Grill the Sandwich: In the same skillet, grill the assembled sandwich over medium heat for approximately 3-4 minutes on each side, or until the bread is beautifully golden and crispy. Gently press down with a spatula while grilling to ensure even browning.
Serve: When the patty melt reaches that perfect golden color, carefully remove it from the skillet. Allow it to rest for about a minute before slicing it in half. Serve immediately and enjoy the blissful combination of flavors.
Notes
Serve with a side of crispy fries or a fresh garden salad. Garnish with fresh herbs for added color.