01 -
In a bowl, whisk mayo, ketchup, mustard, pickle juice, garlic powder, and paprika until smooth. Put aside for later.
02 -
Warm up olive oil in a pan over medium-low. Toss in the sliced onions and cook them until they're soft and caramelized, around 30 minutes. If they dry out or stick, splash in a bit of water and lower the heat. Set aside when done.
03 -
Combine ground beef with Worcestershire, salt, and pepper in a mixing bowl. Form into four thin patties that are about the size of your bread slices.
04 -
Cook the patties in a heated skillet over medium heat until the outside browns. Take them out and set them aside. Clean out the pan.
05 -
Spread a layer of butter on one side of each bread slice. Heat up the skillet and place one slice buttered side down, then stack it with Swiss cheese, a patty, caramelized onions, dressing, cheddar cheese, and another bread slice buttered side facing up. Grill on medium until the outside is crispy and the cheese inside is gooey, flipping once. Repeat for the rest of the sandwiches.