Slice sweet onions thinly. Heat olive oil in a skillet over medium-low heat, add onions, salt, and pepper. Cook for 45 minutes until soft and golden brown.

In a medium bowl, combine mayonnaise, chopped cooked bacon, Worcestershire sauce, and garlic powder. Mix until well combined and refrigerate.

Divide ground chuck into 8 equal balls. Preheat a griddle or skillet over medium-high heat.

Place each meatball on the griddle, smash them flat. Season with kosher salt and black pepper. Cook for 3 minutes.

Flip patties, place a slice of cheese on each burger, top half with caramelized onions. Cook for 45 seconds until cheese melts.

Stack one cheese-topped patty onto another patty. Keep warm on a plate.

Spread baconnaise on the bottom half of each bun, add lettuce, burger stack, tomato, and top half of the bun.

Serve immediately, pairing with crispy fries or salad.