Gently shape the sirloin into four 1-inch-thick patties. Season generously with salt and pepper. In a large cast-iron skillet, heat the oil. Cook the burgers over moderately high heat until deep brown outside and medium-rare within, about 6 minutes per side. During the last 2 minutes, top the burgers with the cheese and cover loosely with foil so the cheese melts. Transfer the burgers to the buns, top with the caramelized onion and serve with a side of lettuce, tomato and pickles.