2) Warm a skillet over a medium heat and fry the bacon until crispy. Remove to a paper towel-lined plate to drain. Keep the skillet on the heat and fry a pinch of the beef in the bacon fat to test seasoning. Adjust the seasoning, if needed, then form 8 even thin burgers, about 0.25cm thick and set aside.
3) Saute the onions in the bacon fat the skillet until tender, 5 mins. Place the onions and bacon in a bowl with the cheese. Form a spoonful of bacon-cheese mixture into a ball, then place in the centre of 4 of the burgers. Top each with another burger and seal the edges.
4) Griddle the burgers over a medium-high heat, flipping once until the desired doneness is reached, approximately 4 mins each side for medium. Serve in the buns with the garnishes. Alternatively, in a medium skillet over a medium-high heat, sear the burgers 4 mins on each side.