Prepare an outdoor grill for medium heat and lightly oil the grate.

Cook bacon in a skillet over medium heat, stirring frequently, for about 5 minutes. Add onion to the skillet; cook and stir until bacon is browned but not crisp and onion is soft. Transfer bacon and onion to a paper towel-lined plate to drain. Season with black pepper.

Combine ground beef and onion soup mix in a large bowl; divide into 8 balls, then flatten into thin patties. Place 1/4 each bacon mixture on 4 patties; top each with 1 remaining patty. Press edges together to seal patties.

Cook on the preheated grill until well done, flipping once, about 15 minutes. Serve on buns.