How to Make It
Cook bacon in a cast-iron skillet over medium, turning often, until just crispy, about 8 minutes. Drain on paper towels. Cut each slice in half crosswise; set aside. Reserve 1 tablespoon drippings in skillet. Add onion rings to skillet; cook over medium, turning occasionally, until soft and caramelized, 10 to 12 minutes. Drain on paper towels; set aside. Remove skillet from heat, and wipe clean.
Mix together beef, mustard, Worcestershire, salt, and pepper in a bowl using your hands until thoroughly combined. Shape into 4 (3/4-inch-thick) patties.
Heat skillet over medium-high; add oil. Add patties; cook until desired degree of doneness, or about 4 minutes per side for medium well (about 160°F). Transfer to a plate; immediately top each patty with 1 cheese slice.
Spread cut sides of bun halves evenly with mayonnaise. Spread top halves evenly with ketchup. Top bottom halves evenly with patties, onion rings, bacon pieces, and pickles. Replace top bun halves.