01 -
Combine mayonnaise, ketchup, sweet pickle relish, and Worcestershire sauce in a small bowl. Mix until fully blended and set aside.
02 -
Melt butter in a large skillet over medium heat. Add thinly sliced onions and cook slowly, stirring often, until deeply golden and sweet, about 15-20 minutes. Remove onions and set aside.
03 -
Divide ground beef into 4 equal portions and form thin, even patties. Season both sides generously with salt and black pepper.
04 -
Cook patties in skillet over medium-high heat, 3-4 minutes per side, until browned and cooked through. Transfer to a plate.
05 -
Butter one side of each rye bread slice. Place 4 slices, buttered side down, in the clean skillet. Top each with Swiss cheese, a beef patty, caramelized onions, and cheddar cheese. Cover with remaining bread slices, buttered side up.
06 -
Grill assembled sandwiches in skillet over medium heat. Cook each side for 2-3 minutes, pressing gently, until bread is golden and cheese is melted.
07 -
Let sandwiches rest 1 minute before slicing. Serve warm with special sauce alongside.