Cook the bacon in a medium cast-iron skillet over medium heat, flipping about halfway through, until crisp and the fat has rendered, about 8 minutes; transfer to a plate. Pour off and reserve the bacon drippings, leaving enough in the skillet to coat it nicely.
Slice 4 rounded chunks of lettuce (4 to 5 inches across) from the top and 3 sides of the head to make "bun" halves; set on a platter.
Shape the ground beef into 2 burgers, each about 4 inches wide and 3/4 inch thick, and sprinkle both sides with salt and pepper. Put the bacon skillet over high heat, add the burgers and cook for 3 minutes; flip and cook for 3 minutes more for medium rare.
Lay 1 lettuce "bun" half cut-side up on each of 2 plates and top with a burger. Spoon 1 tablespoon of the dressing over each burger and top with 1 tablespoon of the onion. Crumble 1 piece of bacon over each burger and top with the remaining lettuce "bun" tops.
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