Cook the bacon in a medium cast-iron skillet over medium heat until it is crisp and the fat has rendered, about 8 minutes, flipping about halfway through; transfer to a plate. Pour off and reserve the bacon drippings, leaving enough in the pan to film it nicely.
From the top and 3 sides of the lettuce, slice 4 round chunks 4 to 5 inches across; set on a platter.
Shape the ground beef into 4 burgers, each about 4 inches across and 3/4 inch thick, and sprinkle both sides with salt and pepper. Add the burgers to the skillet over high heat. Cook for 3 minutes, flip and cook for 3 minutes more for medium-rare.
Lay 1 lettuce "bun" cut side up on each of two 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 buns.
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