1Flatten the ground beef into rectangular patties on parchment paper.
2Season the beef patties generously.
3Dice the bacon into small pieces.
4Turn on the griddle to medium heat.
5Cook the diced bacon on the griddle until crispy, then remove and set aside.
6In a bowl, combine mayonnaise, balsamic vinegar, honey, ketchup, yellow mustard, red pepper flakes, and the cooked bacon pieces.
7Mix well until all ingredients are thoroughly combined to create the baconnaise sauce.
8Slice the onions thinly using a mandoline slicer.
9Place the sliced onions on the griddle and season with salt.
10Cook the onions until caramelized and tender, then add a splash of balsamic vinegar and mix.
11Butter slices of sourdough bread and toast them on the griddle until golden brown and crispy on both sides.
12Cook the seasoned beef patties on the griddle until browned and cooked through, adding bacon bits and American cheese slices on top as they cook to melt the cheese.
13Spread a generous layer of baconnaise sauce on one slice of toasted sourdough bread.
14Place a cooked patty with melted cheese and bacon on top of the sauce.
15Add a generous amount of caramelized onions on top of the patty.
16Top with another slice of toasted sourdough bread.
17Gently press the sandwich to combine the layers.