Preheat oven to 375°F. Arrange bacon on a slotted pan and bake to crisp.
Combine meat with grated onion, horseradish, Worcestershire sauce, salt and pepper.
Preheat a cast-iron skillet over medium-high heat. Add a little oil, 1 turn of the pan, and cook burgers to deep golden and crispy on each side, about 7-8 minutes (burgers should be medium-rare). Remove to a plate.
Add butter to the pan and melt; add garlic and stir 1 minute. Add hot sauce then return patties to pan and turn to coat in Buffalo sauce. Top burgers with cheese, turn off heat and cover pan 1 minute to melt cheese.
On bun bottoms, place lettuce, tomato, onion and bacon then top with cheeseburgers. Smear relish on bun tops and set in place.