Step 1: Caramelize the Onions

Start by caramelizing the onions, as this process takes the longest. Heat 1 tablespoon of butter in a large skillet over medium-low heat. Add the thinly sliced onions to the skillet and cook, stirring occasionally, for about 20-25 minutes until they become golden brown and caramelized. Reduce the heat if they begin to brown too quickly. Once caramelized, set the onions aside.

Step 2: Make the Secret Sauce

While the onions are cooking, prepare the secret sauce. In a small bowl, whisk together the mayonnaise, ketchup, Dijon mustard, Worcestershire sauce, garlic powder, paprika, and pickle relish (if using). Stir until well combined, then set aside for later.

Step 3: Shape and Season the Patties

Divide the ground beef into four equal portions and shape each portion into a thin patty. The patties should be about the size of your bread slices, as they will shrink slightly when cooked. Season both sides of each patty generously with salt and pepper.

Step 4: Cook the Beef Patties

In the same skillet used for the onions, cook the patties over medium-high heat. Sear the patties for about 3-4 minutes per side, until they are cooked to your desired level of doneness (medium or medium-well works well for patty melts). Once the patties are cooked, remove them from the skillet and set them aside.

Step 5: Assemble the Patty Melts

To assemble the melts, start by placing a slice of Swiss cheese on one side of each slice of rye bread. Place one cooked beef patty on top of the cheese, then add a generous portion of the caramelized onions. Spread a spoonful of the secret sauce on the onions, then top with another slice of Swiss cheese and a second slice of rye bread.

Step 6: Grill the Patty Melts

Wipe out the skillet if necessary, then melt 1 tablespoon of butter over medium heat. Place the assembled patty melts in the skillet (you may need to do this in batches) and cook for about 3-4 minutes per side, or until the bread is golden brown and crispy, and the cheese has melted. Add more butter to the skillet as needed to grill the remaining sandwiches.

Step 7: Serve and Enjoy

Once the patty melts are grilled to perfection, remove them from the skillet and let them cool for a minute or two before serving. Cut them in half and serve with your favorite side dishes like fries, chips, or a simple salad.