01 -
Combine mayo, ketchup, relish, and Worcestershire in a small bowl. Keep aside.
02 -
In a pan, heat butter and cook the onions on medium. Stir now and then until they turn golden, about 15-20 minutes.
03 -
Make 4 flat patties from the ground beef, season both sides with salt and pepper, and cook for 3-4 minutes per side.
04 -
Spread butter on bread slices. Stack a slice of Swiss, a patty, some onions, and a slice of cheddar between two pieces of bread.
05 -
Grill the sandwiches on each side for 2-3 minutes until the bread is toasty and the cheese drips.