Grate the frozen butter into a large mixing bowl using a fine box grater. Then add the ground beef to the butter. Mix the meat and butter together with your hands until the butter is well incorporated. Then, form 4 equal-sized patties, careful not to over work the patties. Season the patties on both sides with salt and pepper.

Add the rendered bacon fat or neutral oil to a large frying pan over medium high heat. When the fat just starts to smoke, add the patties to the pan. Allow them to cook for 2-3 minutes before flipping them. Add the cheese to the burger and cook for 2-3 minutes or until desired doneness is achieved. *Option - The cheese can also be melted using an inexpensive cheese melting pan, then added to the burger for some extra gooey cheese.

Remove the patties from the pan and place them on a cooling rack to rest.

NOTE* The burgers will be dripping; put double layer of paper towel under the cooling rack helps prevent a messy clean up.

Add the sliced onions into the hot pan and allow them to cook down in the rendered fat for 2 minutes, until they start to soften.

Then, deglaze the pan with the beef stock, and add in the brown sugar and salt. Move the onions around the pan often, until dark and caramelized, about 5 more minutes.