First, lay out a large piece of wax paper. Separate the beef into 8 pieces, and roll each piece into a ball. Flatten out the beef balls using your hands, then lay them on the wax paper, forming them into patties. You want them to be thin- about 4 inches in diameter- or a little bigger than the bun. Repeat with all 8 beef balls and season them with salt and pepper. Cut them into squares and stack them on a plate.
Lightly oil a large skillet and heat on Medium-High for a few minutes before adding the patties. Place 2 patties on the skillet with the seasoned side down, and season the top with salt and pepper and 1 tbsp of mustard. Cook these patties until browned- about 2 to 3 minutes. Flip them and cook them for about another minute.
While still on the skillet, add sliced cheese to both patties. Then add 1 heaping tbsp of caramelized onions onto one patty, and stack the other patty on top. Remove the stack from the pan, and repeat with the remaining patties.