Slice 8 large rounds from the edges of the head of iceberg to create buns.
In a large skillet over medium heat, cook bacon until crispy. Transfer bacon to a paper towel-lined plate to drain, reserving bacon fat in pan.
Add onion slices and cook until tender, about 3 minutes per side. Set aside red onions and wipe skillet clean.
Return skillet over medium-high heat. Shape ground beef into 4 large burger patties. Season both sides of patties with salt and pepper and add to skillet. Cook until seared on both sides and cooked to your liking, about 4 minutes per side for medium. Top each burger with a slice of cheese, then cover skillet with a lid and cook until cheese has melted, about 1 minute more.
To build each burger, top one iceberg round with the cooked cheeseburger, a slice of bacon, and a tomato slice, then drizzle with ranch. Top with second iceberg round. Repeat with remaining ingredients and serve.