Caramelize the onions

• Slice the onions lengthwise.
• In a skillet over medium-low heat, add the olive oil and part of the butter.
• Once the butter is melted, add the sliced onions and stir to coat.
• Cook slowly, stirring occasionally, until softened and browned, then add a pinch of salt and cook a couple more minutes. Set aside.

Form the beef patties

• Divide the ground beef into 8 portions of about ¼ pound each.
• Roll each portion into a ball.

Smash and cook the burgers

• Heat the cast iron skillet over medium-high heat and add the neutral oil.
• Once the oil is hot, add a beef ball and smash it down very thin.
• Season with salt, black pepper and garlic powder.
• Cook a couple of minutes until a good crust forms, then flip.
• Add cheese, cover and cook until the cheese is melted and the burger is cooked to preference. Set aside and repeat with the remaining patties.

Make the burger sauce

• In a bowl, combine the mayonnaise, ketchup, dill relish, white vinegar, Worcestershire sauce, white sugar, garlic powder and black pepper.
• Mix well, then add salt to taste if desired.

Assemble the patty melts

• Spread a layer of burger sauce on a slice of bread, then add cheese.
• Top with the patties and onions, more burger sauce and cheese, then cover with a second slice of bread.

Toast the patty melts

• In a skillet over medium-low heat, melt some butter.
• Add the assembled patty melt, press down and toast on both sides until golden brown.