In a large nonstick skillet, fry the bacon until crisp. Remove the bacon from the pan and drain on a paper towel-lined plate saving the bacon grease. Keep 1 tablespoon of the grease in the skillet and set aside 2 tablespoons for later. Finely chop 1/2 of one of the onions and saut? it in the 1 tablespoon grease for 3 minutes; add to the bacon and let cool.

Meanwhile mix the ground sirloin, Worcestershire sauce, steak seasoning and salt in a large bowl. Stir in the bacon mixture. Form into 4 patties. Heat the skillet over medium-high heat. There should be enough grease left in the pan to fry the burgers. Cook the burgers 4 minutes per side for medium-rare and 6 minutes per side for medium-well. Top each burger with 1/4 of the cheese; cover the pan until the cheese melts. Serve the burgers topped with the saut?ed onions.

Thinly slice the remaining onion. Heat the remaining 2 tablespoons bacon grease in the skillet and saut? the onions over medium heat, covered, about 10 minutes or until golden brown. Stir occasionally. Season with salt. Remove the onions from the skillet and keep warm while cooking the burgers.