Divide the beef into 4 portions and form each one into a log that is similar in length to the croissants. Don't overwork the meat - they don't need to be perfect. Leave at room temp to one side.
Add the bacon to a large heavy-based pan and place over medium heat. Fry until crisp with the fat rendered, then place to one side. You want a couple of tsp worth of fat left; discard/wipe out if there's a lot more (just eyeball it).
At this point, I recommend turning the extractor fan on and consider opening a window. Turn the pan heat to high, and once smoking hot, add two portions of the meat. Quickly add a piece of baking paper over each portion and use a spatula with your hand to very firmly press down until they flatten. You want them relatively thin (5-7mm), but not so thin they break apart.
Remove the baking paper and generously season with salt and black pepper. Leave for 90 seconds, then use the spatula to flip them both over. Season again with more salt and pepper (more modestly this time), then top with cheese. Leave for another 90 seconds, then remove them from the pan. Don't press down on the patties at any point after you've smashed them otherwise the juices will pour out. Remove the patties and place to one side, then repeat with the second portions. You won't need extra oil.
Use a bread knife to carefully slice open the croissants and place on a large baking tray. Place under the grill/broiler until crisp (be careful as they burn quickly).
Stack the burgers with sauce, patty, bacon and onions, then more sauce on the top side. Fold over, then tuck in and enjoy!