1️⃣ Prepare the Secret Sauce:
In a small bowl, mix together 1/2 cup mayonnaise, 2 tablespoons ketchup, 1 tablespoon sweet pickle relish, and 1 teaspoon Worcestershire sauce. Stir until smooth, then set aside to let the flavors blend.

2️⃣ Caramelize the Onions:
Heat a skillet over medium heat and add a tablespoon of butter. Add the thinly sliced onions, season with a pinch of salt, and cook for 15-20 minutes, stirring occasionally, until caramelized and golden. If the onions start to stick, add a splash of water or broth to deglaze the pan. Remove from the skillet and set aside.

3️⃣ Form and Cook the Beef Patties:
Divide the ground beef into four equal portions, forming thin patties slightly larger than the bread slices (to account for shrinkage). Season both sides with salt and pepper.
In the same skillet, cook the patties over medium-high heat for 3-4 minutes on each side or until they reach your preferred doneness. Set aside.

4️⃣ Assemble the Patty Melts:
Lay out the rye bread slices. Place Swiss cheese on four slices and cheddar cheese on the other four. Top each Swiss cheese slice with a beef patty, then add a generous portion of caramelized onions. Place the cheddar-topped bread slice on top to complete each sandwich.

5️⃣ Grill the Patty Melts:
Wipe out the skillet and add a tablespoon of butter. Place the assembled sandwiches in the skillet (working in batches if necessary). Grill for 2-3 minutes per side over medium heat, pressing down gently with a spatula until the bread is golden and the cheese has melted. Add more butter as needed for even toasting.

6️⃣ Serve with Secret Sauce:
Once the patty melts are golden and the cheese is melted, remove them from the skillet. Let rest for a minute, then cut each sandwich in half. Serve with a side of the secret sauce for dipping or spread it directly on the sandwich for added flavor.