Form each ball of ground beef into a flat patty. Place 1 1/2 Tbsp. of cheese onto the center of each and form the meat around the cheese. Season liberally with salt and pepper on both sides. Heat the oil in a skillet over medium-high heat until hot. Cook the burgers 5 to 6 minutes on each side. Serve each on a bun with 1 Tbsp. mustard, 2 slices tomato, 1 lettuce leaf and a pinch of onions.