Place onions, butter, and salt in a large nonstick skillet over medium-low heat and cook, stirring frequently until the onions are golden and caramelized, about 20-25 minutes. Transfer to a bowl and set aside.

While the onions are cooking, cut the bacon strips in half and place in a medium skillet over medium-low heat. Cook until bacon starts to release its fat and curl. Turn each slice with tongs to cook on the other side. Continue to flip and cook to desired crispiness, then place on a plate lined with paper towels and set aside.

In a non-stick skillet over medium heat, add the burger patties and cook until browned on one side, about 2 minutes. Flip and cook until browned and the internal temperature is 165°F (about 2 minutes). During the last minute of cooking, add 1 oz. crumbled blue cheese to each patty. Remove from heat.

Place each burger patty between each half of a bun, top with caramelized onions, bacon, butter lettuce, and any other toppings, and enjoy!