1️⃣ Prepare the Secret Sauce
In a small bowl, combine mayonnaise, ketchup, sweet pickle relish, and Worcestershire sauce. Stir until smooth. Cover and refrigerate while preparing the patty melts.

2️⃣ Caramelize the Onions

Heat 1 tbsp of butter in a large skillet over medium heat.

Add the sliced onions and cook, stirring occasionally, for 15-20 minutes, or until golden brown and caramelized.

Remove the onions from the skillet and set them aside.

3️⃣ Shape and Season the Patties

Divide the ground beef into 4 equal portions and shape each into a thin patty, slightly larger than the size of the bread slices (they’ll shrink while cooking).

Season both sides of each patty with salt and pepper.

4️⃣ Cook the Patties

Using the same skillet, cook the patties over medium-high heat for about 3-4 minutes per side, or until cooked through.

Remove the patties from the skillet and set them aside.

5️⃣ Assemble the Patty Melts

Butter one side of each slice of rye bread.

On a clean skillet or griddle over medium heat, place 4 slices of bread butter-side down.

Layer the ingredients as follows: Swiss cheese, a cooked beef patty, caramelized onions, cheddar cheese, and top with another slice of bread (butter-side up).

6️⃣ Grill the Sandwiches

Cook the patty melts for 2-3 minutes per side, pressing gently with a spatula, until the bread is golden brown and crispy, and the cheese is fully melted.

7️⃣ Serve

Remove the patty melts from the skillet and let them rest for 1 minute. Slice each sandwich in half.

Serve immediately with the secret sauce drizzled on top or on the side for dipping.