Thinly slice the sweet onions and heat a large skillet over medium-low heat. Add olive oil along with a pinch of salt and black pepper. Cook for about 45 minutes, stirring occasionally, until golden brown and soft.

In a medium bowl, combine mayonnaise, chopped cooked bacon, Worcestershire sauce, and garlic powder. Mix until creamy and set aside.

Divide the ground chuck into eight equal portions, rolling them into balls. Preheat a griddle until sizzling hot. Smash each ball down flat, seasoning with kosher salt and black pepper.

Let the patties sear for 2-3 minutes until a crust forms. Flip each patty, adding a slice of American cheese on top. Add caramelized onions on half the patties and cook until the cheese melts.

Slice the brioche buns and spread baconnaise sauce on both sides. Stack two cooked patties together with melted cheese and onions, then layer with lettuce and tomato before placing the top bun.

Allow the assembled burgers to rest for a couple of minutes to let flavors meld.