In a large frying pan over medium-high heat, add bacon. Cook until crispy and remove to a paper towel covered plate and keep warm. Reserve bacon grease in the pan.
Preheat oven to 450degF.
Combine ingredients for patties and form into 12 equal balls. Place two to three balls into the hot pan (with bacon grease) and use a wide flat spatula to press each down into thin patties (¼ inch thick). Cook for 3 minutes then flip and press down again. Let cook for 2 minutes then top with each with a slice of cheese and let cook another minute. Stack two patties together. Repeat with remaining balls.
Open buns and lay them out on a parchment paper covered baking sheet. Spray buns with thin layer of cooking spray. Place in oven to toast for 5-7 minutes.
Build the burger - smear top and bottom bun with mayo. Add double cheeseburger, then 3 pieces of bacon. Top with lettuce, tomato, and red onion. Place top bun and press down slightly to help hold it all together.