01 -
Heat olive oil in a skillet over medium-low heat. Add sliced onions, season with kosher salt and freshly ground black pepper, and cook, stirring occasionally, for 20 to 25 minutes until onions are soft and golden brown. Set onions aside.
02 -
In a mixing bowl, combine ground beef, ketchup, Worcestershire sauce, onion powder, kosher salt, and black pepper. Mix just until combined, then shape into 4 patties slightly larger than the rye bread slices.
03 -
Melt 2 tablespoons unsalted butter in a cast-iron skillet over medium heat. Add beef patties and cook for 4 to 5 minutes per side until well browned and fully cooked through. Remove patties from the skillet and set aside.
04 -
Wipe out the skillet and melt the remaining butter. For each sandwich, place one rye bread slice on a work surface, top with a beef patty, a generous portion of caramelized onions, and two slices of Swiss cheese. Top with a second bread slice. Grill each assembled sandwich in the skillet until bread is golden and cheese is melted, about 3 minutes per side. Serve immediately.