Mix all ingredients together until well combined and set aside.
Divide your ground sirloin into 4 even parts and roll into balls. Set aside.
Slice your onion VERY thin. If you have a mandolin slicer use this on the thinnest setting. If not, practice your chef skills and use your sharpest knife to carefully slice your onion as thin as possible--the thinner the better for this recipe!
Place a large non-stick saute pan or cast iron (or if you have a griddle, use that!) on HIGH heat. Turn on your vent fan if you have one, otherwise, try to open a window if possible. There will be some smoke during cooking.
Add the burger patty balls to your pan and carefully top each with a nice handful of the onions. Then using either a burger press or very sturdy spatula (I've also improvised and used the bottom of a small saute pan!), press firmly down on the top of each patty ball until it spreads very thin and is about 1 inch larger in diameter than your buns (because they will shrink when cooking).
Season the top with kosher salt and cook for about 1 min and 30 seconds, flip, add the cheese, cover and cook for another 1 min and 30 seconds.
Optional: if you prefer your buns toasted, then add some spreadable butter or cooking spray to each side and place down on your pan for just a few seconds until they begin to brown. Remove and set aside.
Remove the patties and place on your burger buns. I recommend waiting a 1-2 minutes to let the juices from the burger seal before enjoying!