Preheat your cast iron skillet over medium-high heat.
Peel the onion and slice horizontally into thick rounds, keeping the rings intact.
Season the onion rounds generously with salt and pepper.
Place the onion rounds in the skillet and cook for 3-4 minutes on each side until tender and caramelized. Add one slice of cheese. Set aside.
Divide the ground beef into four equal portions and shape into thin patties. Season the patties with salt and pepper on both sides. Place the patties in the skillet and cook for 2-3 minutes on each side until desired doneness is reached. (Optional: you can fry the patties in mustard)
Place a slice of cheese on top of two patties and allow it to melt slightly. Place one grilled onion round on a plate, followed by a cheese-topped patty.
Add optional toppings like lettuce, tomato, and pickles. Place another grilled onion round on top, followed by the remaining cheese-topped patty.
Finish with a final grilled onion round on top to complete the "bun". Serve immediately and enjoy the mouthwatering goodness!