Step 1: Caramelize the Onions

1. Heat 1 tbsp of oil in a skillet over medium-low heat.
2. Add the sliced onions and cook, stirring occasionally, for 20-25 minutes until caramelized.
3. Remove from heat and set aside.

Step 2: Prepare the Beef Patties

1. In a bowl, combine the ground beef, Worcestershire sauce, garlic powder, salt, and black pepper.
2. Divide into 4 equal portions and shape into thin, bread-sized patties.
3. Heat a cast iron skillet over medium-high heat and cook patties for 3-4 minutes per side.
4. Remove from heat and let rest.

Step 3: Assemble the Sandwich

1. Butter one side of each rye bread slice.
2. On the unbuttered side, place cheddar cheese, a beef patty, caramelized onions, and Swiss cheese.
3. Top with another slice of bread, buttered side facing out.

Step 4: Grill the Patty Melt

1. Heat a skillet over medium heat.
2. Cook sandwiches for 3-4 minutes per side, pressing lightly with a spatula.
3. Flip carefully and cook until cheese is melted and bread is golden brown.
4. Let rest for 1-2 minutes, then slice and serve hot.