1. Caramelize the Onions:

   • In a large skillet over medium heat, melt 4 tablespoons butter.
   • Add the sliced onions, season with salt and pepper, and cook over medium-low heat for 15-20 minutes, stirring occasionally until soft and caramelized. Remove from heat.

2. Prepare the Burger Patties:

   • In a large mixing bowl, combine ground beef, Worcestershire sauce, garlic powder, 1 teaspoon salt, and ½ teaspoon black pepper.
   • Mix thoroughly and divide into six equal portions.
   • Shape each portion into a thin, oval-shaped patty slightly larger than the bread slices (they will shrink while cooking).

3. Toast the Bread:

   • Lay out 12 slices of bread and spread 1 teaspoon yellow mustard on six of them.

4. Cook the Patties:

   • Heat a griddle or large skillet over medium heat.
   • Cook the patties for 3 minutes per side.
   • After flipping, top each patty with caramelized onions, then place one slice of American cheese and one slice of Swiss cheese on each patty.
   • Cook for a total of 6 minutes until fully cooked and cheese is melting.

5. Assemble & Toast the Patty Melt:

   • Transfer each loaded patty onto the mustard-coated bread slices.
   • Wipe the griddle clean, then melt 2 tablespoons butter on the griddle.
   • Place assembled sandwiches on the griddle and top with the remaining bread slices.
   • Toast for 2-3 minutes on one side until golden brown.
   • Add the remaining 2 tablespoons butter, flip, and toast the other side for another 2-3 minutes.

6. Serve & Enjoy:

   • Transfer patty melts to a cutting board, slice in half, and serve immediately.

Enjoy your delicious, cheesy, crispy Patty Melt! 🍔🧀😋