Press each ground beef portion onto the tortillas to cover one side evenly and completely. Beef should be about ¼ inch thick at most. Add diced onion to ground beef side and sprinkle with kosher salt to taste.
In a preheated skillet over medium heat, or on a griddle set to 325ºF, place the tortilla beef side down and cook until the meat is done. Remove excess grease from around the tortilla and flip it onto the tortilla side. Add cheese to the burger side and turn off heat.
When the cheese is melted, transfer the taco onto a paper towel to and allow to cool for one minute.
Place taco on a plate and top with Big Mac sauce and shredded lettuce. Serve immediately.
