Warm olive oil in a large skillet over medium heat. Add onions, season with salt and stir to coat with oil. Cook, stirring occasionally until onions are brown and caramelized, about 20 minutes. Stir in balsamic vinegar and cook for another 1-2 minutes. Remove from heat and set aside.
In a small bowl mix mayonnaise, barbecue sauce and liquid smoke until creamy. Refrigerate until ready to use.
In a large bowl mix ground beef, cheese, bacon, worcestershire, liquid smoke, salt and pepper until combined.
Form into 4 equal sized patties.
Preheat grill or skillet with olive oil over medium-high heat.
Add patties and cook for about 6-7 minutes. Flip, and cook for another 6-7 minutes until cooked thoroughly.
Assemble burgers on toasted buns topped with caramelized onions and campfire sauce.