For the BBQ sauce: Combine all the ingredients in a medium saucepan and bring to a simmer over medium heat. Cook for 3 minutes then remove to cool.
Season the burger patties with salt. Heat a large skillet
Lightly oil the pan and place the burgers on the pan. Once juices start to pool at the bottom of the burger use a smash burger press or large metal spatula to smash the burger until it is SUPER THIN! Add the onions.
Once you see juices pooling on the meat flip the burger. Add the cheese and place a bottom bun on two of the four patties. As soon as the cheese is melted pull them off.
Flip the bottoms over and add the other burger to make two doubles. Top with more BBQ sauce and shredded lettuce.