Place diced bacon in a cold pan. Cook over medium-low heat for 20 minutes or so, stirring occasionally, until bacon is browned and crispy. Transfer bacon to a paper towel lined plate to blot excess grease and save rendered bacon fat for use another time (seriously, please save your bacon fat).
Mix ground beef with salt, pepper and worcestershire only until seasonings are evenly distributed and no more. Portion into 8 4oz patties. Place 2 oz brie and some of the bacon in the middle of one patty and cover it with another patty, pinching the edges to seal. Repeat with all patties until you have 4 stuffed burgers. Place in fridge for 30 min prior to cooking to firm up.
Preheat your grill or cast iron pan (indirect heat with your grill). Sear burgers 5-7 minutes per side plus an additional 2-3 minutes on the hot side of the grill to get some char. Alternately, if you're cooking in your cast iron, transfer to the oven after searing for another 5 minutes to finish. Remove burgers from grill or pan and set aside for 5 minutes. In the meantime, place onion slices on grill or in pan and cook until charred and soft. Serve burgers on toasted buns with pickles, grilled onions and burger sauce.
Mix all ingredients together and store in the fridge until ready to use. To make the chopping easy i just put a few onion slices and pickles in the food processor (I have a mini one) to get everything finely chopped.