Begin by forming 2 pounds of ground chuck into tightly packed 1/3 pound balls. Place the meat balls on a tray and refrigerate for at least 30 minutes to help them firm up.

Using a mandoline, thinly slice the white onions and soak them in cold water for about 10 minutes, then refrigerate.

Preheat a skillet over medium heat, add chopped bacon, and sauté until crispy and golden, about 5-7 minutes. Drain on paper towels.

In a mixing bowl, combine mayo, ketchup, mustard, chopped bacon, Worcestershire sauce, apple cider vinegar, all-purpose rub, and red pepper flakes. Mix and refrigerate the Baconnaise sauce.

Preheat a griddle over medium-high heat for about 5 minutes and lightly brush it with avocado oil.

Place burger balls on the hot griddle, smash each ball flat into a patty, season, and cook for 3-4 minutes. Flip, add mustard, top with cheese, and cook 3 more minutes.

While burgers cook, toast the burger buns cut-side down on the griddle until lightly golden.

Assemble by spreading Baconnaise sauce on the bottom half of each toasted bun, stacking two smash burgers, adding more sauce, and covering with the top bun.