Thinly slice the yellow onion into half-moons.
Melt the ghee in a small skillet over medium-low heat, add the onions, and cook slowly until soft and deep golden brown, about 15 minutes.
While onions caramelize, divide the ground beef into two 3-ounce portions and shape them into very thin, wide patties.
Season both sides of the patties evenly with sea salt, black pepper, and garlic powder.
In a separate cast-iron skillet over medium heat, cook the bacon until crispy, then set aside on a paper towel to drain.
Wipe out excess bacon grease and add the avocado oil to the same skillet, increasing heat to medium-high.
Place the beef patties in the hot skillet and sear for 3 minutes per side until a dark crust forms.
Cut the cheddar cheese slice in half and place one half on each patty during the final minute of cooking, covering the pan briefly to melt the cheese.
Lay out the butter lettuce leaves as your base, stack the two cheesy patties on top, add the bacon slices, and finish with the caramelized onions before wrapping the lettuce tightly around the burger.