Form Patties: Shape the ground chuck into 1/3 lb balls. Refrigerate for 30 minutes.
Prep Onions: Use a mandoline to slice onions thin. Pat dry and refrigerate.
Cook Bacon: Fry or bake until crispy. Let cool and chop finely.
Make Sauce: Combine mayo, ketchup, mustard, chopped bacon, Worcestershire, vinegar, rub, and pepper flakes. Mix well and chill.
Heat Griddle: Preheat a griddle or skillet to medium-high. Add avocado oil.
Smash Burgers: Add meatballs to the griddle, top with onions, and smash down flat. Season well.
Flip and Cheese: After 3–4 minutes, spread mustard on top, flip, and add cheese. Cook another 3 minutes.
Toast Buns: Toast the buns cut side down on the griddle until golden.
Assemble Burgers: Spread Baconnaise Sauce on bottom bun, stack patties, top with more sauce, and finish with top bun.