Preheat a skillet over medium heat.
Lightly toast both halves of the hamburger bun, face down. Set aside.
Separate the beef into two even portions, and form each half into a thin patty slightly larger than the bun.
Lightly salt each patty and cook for 2 to 3 minutes on the first side. Flip the patties over and place two slices of cheese on top of each one. Cook for 2 to 3 minutes.
Build the burger in the following stacking order from the bottom up: bottom bun, dressing, tomato, lettuce, beef patty with cheese, onion slice, beef patty with cheese, top bun.