Light your grill and set up for direct medium heat. Place a cast iron pan / griddle over a portion of the hot coals.
While the grill is preheating, form the meat into four equally sized patties. Season both sides with salt and pepper. Then, slice the buns and drizzle the cut side with olive oil.
Place the burgers on the grill, directly over the heat. Grill for about three minutes on each side for a medium to medium rare burger. Place the buns, cut side down on the grill for the last 45 seconds the burgers are grilling. Then, remove the buns and burgers, placing a burger on each of the bottom half of the buns.
Place cheese slices on the preheated cast iron. Sear them for 30-45 seconds on each side, flipping once.
Dress with chimichurri and red onions, top with toasted bun and serve hot.